Hung Le

Vu

Damien Foures

Vincent Albert

ProDEVS W an EventpDriven Modeling and Simulation Tool for Hybrid Systems using State Machines

Keywords:

T his paper int roduces a new event pdriven modeling and simp ulat ion t ool for t he simulat ion of hybrid syst emsG T he parp t icularity of t his software called ProDEV S lies in it s graphp ical language t o define model component s behaviourG T his graphical language cust omizes st at e machines for DEV S and quant ized based numerical met hodsG In t his paper4 synt ax and operat ional semant ic of t he language are explained4 and a mapping from DEV S t o t his language is illust rat ed across two simple examples in discret epevent and cont inuous dop mainG Finally a complet e hybrid syst em is modeled and simulat ed t o show t he usability and t he e ciency of t his modelG

INTRODUCTION

DEV S .Discret e EVent Syst em Specificat ion6 [q(] is a general formalism for specifying modular and hierarchical model of dynamic syst emsG A DEV S specificat ion is exep cut ed by an event pdriven simulat or which ensures t he schedulp ing of t imed event s and increases t he simulat ion clock t o t he t ime of next event G A lgorit hms and met hods t o implement an event pdriven simulat or for DEV S models are also given in [q(]G T here are numerical met hods like t he Quant ized St at e Syst em .QSS6 family [x] t hat have shown t o e cient ly app proximat e ordinary di erent ial equat ionsG QSS is based on st at e space discret izat ion4 also called quantization4 rat her t han t ime discret izat ion given by convent ional numerical inp t egrat ion met hodsG W it hin QSS4 a quantization function maps realpvalued numbers ont o a discret e set of real values also called quantization levels [x]G A cont inuouspt ime sysp t em is t hen approximat ed by comput ing t he required t ime for a st at e variable t o reach t he next levelG T his t echnique has a st raight forward represent at ion in DEV SG DEV S is even considered [q] as t he most general forp malism since ot her discret e event languages such as Pet ri Net s and St at e Chart s but also discret e t ime syst ems can be seen as part icular cases of DEV SG M oreover DEV S provides a unified framework for represent ing hybrid syst ems which combines discret e and cont inuous dynamics and t he usage of QSS based int egrat ion met hods are not iceably e cient t o simulat e hybrid syst ems due t o t heir ability t o handle disp cont inuit ies [q]G Taking int o account t hese remarks4 DEV S is a good candidat e for modern modeling and simulat ion environment G T here is a huge variety of t ools which support DEV SW DEV SJAVA [

qx]4 aDEV S []]4 CD3 3 [qI]4 PowerDEV S [q]4 JA M ES I I [q)]4 V LE [qz]4 PyPDEV S [qq]4 DEV SpRuby [(]G

A comparison of t his software list is proposed in [)]G If most of t hem have a Graphical User Int erface4 none of t hese t ools are able t o support at t he same t ime a graphical synt ax for at omic component descript ion and t he ment ioned numerical int egrat ion of ordinary di erent ial equat ionsG PowerDEV S is hybrid syst em orient ed but at omic component s are defined in C codeG In t his cont ext 4 we design and implement ProDEV S4 an int egrat ed modeling and simulat ion environment orient ed t o hybrid syst ems based on discret epevent simulat ion t heory and DEV SG We define a language which specializes St at e M achine for building4 animat ing and simulat ing graphical DEV S specificat ionG We implement an event pdriven simup lat ion engine according t o t he algorit hms and t he met hods previously ment ionedG T his idea is mot ivat ed by t he t rend for using highplevel specificat ion language rat her t han code for dynamic syst em designG T he next sect ion gives an overview of t he t ool and it s feap t uresG Sect ion x int roduces t he synt ax and t he semant ic of a ProDEV S modelG Sect ion ( shows t he mapping between a DEV S at omic component and a ProDEV S St at e M achine for discret epevent syst em and cont inuous syst emG Sect ion ) describes an example t o illust rat e t he use of t his model in hybrid simulat ionG Sect ion [ gives conclusions and perspecp t ivesG

TOOL FEATURES

Our software is implement ed in Java and it is based on Figure 1 shows a screenshot of the editor. It gives users an easy navigation within the hierarchical levels of the model and allows simulating the model at different level of the hierarchy. The diagram panel in the center allows editing models. There are two type of pallets if it is coupled component or atomic component. A pallet for the description of a block diagram with port and connector is given for the definition of a coupled component and a pallet for ProDEVS State Machine description is proposed when editing an atomic component. We use the JGraphX library for graphical design of the model.

The WEST region contains a project explorer, a model explorer and a library of components. Components can be drag and drop directly from the library to the diagram (import) or from the diagram panel to the library (export). The components of the library are stored in XML files.

The EAST region contains a properties tab for specifying information of models such as initial values of its variables and its parameters. The simulation tab is used to specify the simulation time and the execution algorithm. Two simulation algorithms are available: classic or parallel [START_REF] Zeigler | Theory of Modeling and Simulation[END_REF]. In classic simulation only one event at a time can occur. In parallel simulation, more than one event can be received by a component at the same time. In case of non-determinism, the receiver component randomly chooses an event to execute. A complete set of mecanisms (priorities, guards) allows to resolve conflicts in the execution and improves the semantic of our language.

The simulation can be performed in step-by-step mode, where each variable value can be observed. At the end of the simulation, a new frame gives the plots for selected ports or variables. The bundle for the visualization of trajectories is based on the JFreeChart library.

For a sake of performance, the simulation engine executes compiled code. A specific bundle for loading the model is called when a new simulation is started. For each atomic component, this bundle generates a .java file that is compiled and executed. Compiled code improve significantly the performances compared to interpreted code that needs parsing boolean and arithmetic expressions. However, the user can access the source file to introduce more complex treatments like loops that can not be expressed graphically until now.

A model verifier is used to check static properties on the model. This properties are about the correct construction of the model (typing, valid expression, non-initialised variable,...). Verification result is given as warning and error in the console tab.

We finally design a ProDEVS model transformation to extended TPN (Timed Petri Nets) with data handling called (TTS) Time Transition Systems [START_REF] Berthomieu | Model checking bounded prioritized time petri nets[END_REF] to generate a finite representation for the accessible states of a ProDEVS model. We consider the Finite and Deterministic subset of DEVS [START_REF] Hwang | Qualitative verification of finite and real-time devs networks[END_REF], classic DEVS and parallel DEVS. This feature is no further explain here and will appear in another paper.

PRODEVS MODEL

We create a customized component-based StateMachine model that supports DEVS syntax and semantic. In substance, we reuse and specialize many concepts of UML 2.4 (Unified Modeling Language), including packages Behav-iorStateMachines, Communications and Kernel. This section gives a description of this model.

Model structure

The abstract syntax for the structure of a ProDEVS model is given by the UML class diagram figure 2. A ProDEVS model compound (composition link) of exactly one component which is either atomic or coupled (inheritance links). A component that is coupled or atomic consists of input or output ports (portDirectionKind attribute). An atomic component is associated with a DEVSStateMachine describing its behavior. Coupled component is composed of one to several atomic or coupled components and zero to several connectors. A port must be typed (the class Port inherits from the class TypedElement). A port acting as a source is connected to zero or more ports acting as a target via connectors. A port that serves as a target is the destination of zero or more ports acting as source via connectors (associations between classes Port and Connector ). A connector can only be associated with two and only two ports at a time. A connector either coupling an output port of a component to an input port of another component (IC) or coupling an output port of a component with an external output (EOC) or coupling an external input to an input port of a component (EIC).

TypedElement defines the type of value DataType that is a comprehensive list of literal values: boolean, double, integer and float. PortDirectionKind is a comprehensive list of literal values: in, out. ConnectorKind is a comprehensive list of literal values: IC, EOC, EIC.

We define constraints to check the correctness of the model. Examples of constraint are given through this section in natural language and in Object Constraint Language (OCL):

If the connector is of type IC, source port must be of type out and the target port must be of type in. In addition, source port and target port must belong to different sub-components of the same current coupled component.

ProDEVS StateMachine

Syntax.

The abstract syntax for a ProDEVS StateMachine is given by the UML class diagram figure 3.

A ProDEVSStateMachine is a set of states-transitions that defines the behavior of an atomic component. A phase is a specific parameter that represents a period in the life of the atomic component where it is expected some events to occur (input event or time event). It has a set outgoing of transitions departing from this phase, a set incoming of transitions entering this phase and a timeAdvance attribute to specify the time in which the system as to stay before triggering the output and the internal transition function (a time advance value can also be given by a variable of the component).

A phase with a time advance equal to infinity must not have an outgoing transition of kind internal and must have at least one outgoing transition of kind external. A phase with a time advance not equal to infinity must have at least one outgoing transition of kind internal not(self.timeAdvance.val <> infinity) implies exists(t | self.outgoing->t.kind <> internal)

A transition is an oriented relationship between a source phase and a target phase. It is associated to :

• a trigger which specifies an input that may fire an external transition in case of external event or an output in case of time event (time elapsed in a phase is reached).

• a guard which provides a fine-grained control over the firing of the transition using boolean expression. The guard is evaluated when an event occurrence, external event or time event, is dispatched by the state machine.

If the guard is true at that time, the transition may be enabled, otherwise, it is disabled. A guard constrains a set of properties1 of the component.

• an action which specifies an optional assignement to be performed onto property when the transition fires.

• a source which designates the originating phase of the transition.

• a target which designates the target phase that is reached when the transition is taken.

Operational semantic.

The system is in phase ϕ at a given time and must be in that phase for a period e = ϕ.timeAdvance, if no external event occurs. When the time e has elapsed without any external event has occurred, the system triggers a time event and calculates and propagates the output. Then, for each outgoing internal transitions the guards are assessed. The system triggers one of those transitions which are enabled. If instead, an external event occurs on the input before the expiration of e, the system triggers the corresponding external transition, if it is enabled. In any case, the system reaches a new phase ϕ ′ = ϕ.outgoing.target for a period defined by ϕ ′ .timeAdvance, the actions associated to the triggered transition are computed and the same algorithm is applied.

When ϕ.timeAdvance = ∞, it means that it is a passive phase and only an external event will leave the phase. When ϕ.timeAdvance = 0, it means that this is a transient phase so the output computation and the internal transition triggering are immediately performed.

In ProDEVS, like in DEVS, communications are weak synchronous, i.e., non blocking with (possible) message loss. If both sender and receiver are ready to communicate, the output event is converted into an input event which is instantly received. If the receiver is not ready, the message is lost.

As a result of coupling of concurrent components, there may be multiple components with simultaneous events. Thus, there may be multiple components which are candidates for the next internal state transition. Such components are called imminents in the DEVS terminology. In classic DEVS, a Select function is added at the coupled component that allows to select the component to execute among a set of imminent components. In parallel DEVS, every imminent components are executed. A component may receive a bag of inputs. A confluent transition function is added to refine a transition in case of simultaneous time event and external event.

A DEVS system is deterministic [START_REF] Posse | Modelling and Simulation of Dynamic Structure Discrete-event Systems[END_REF] as the current state of the system is completely determined by its previous state, the input (if any) and the time elapsed since the last transition or the time advance. In other words the output is completely determined by the timing of the input events [START_REF] Posse | Modelling and Simulation of Dynamic Structure Discrete-event Systems[END_REF]. A ProDEVS system is indeterministic if more than one transition is enabled at the same time. The user can resolve indeterministic behaviour by use of priorities among a set of transitions. If two enabled transitions have the same priority, the transition to trigger is selected randomly.

The label for internal/output function and external transition function are defined by the following BNF expression:

<transition> ::= [<guard>] <trigger> '/' [<action>]
The details of the syntax for the trigger event are defined by different kind of events: <trigger> ::= <time-event> | <external-event> • output events (at time event) are denoted by ! followed by the name of the triggered output port, followed by an assignment specification:

<time-event> ::= [!<name> = <output-assignement>]

• external events are denoted by ? followed by the name of the triggering input port:

<external-event> ::= ?<name>

There are no assignement on port when the trigger is of kind externalEvent.

self.kind <> externalEvent implies self.output->isEmpty()

There is an assignement on output port when the trigger is of kind timeEvent.

self.kind <> timeEvent implies self.output->notEmpty() and self.port = self.output.assignedPort

PRODEVS IN PRACTICE

This section describes how a ProDEVS State Machine can describe a DEVS atomic component for discrete-event system and continuous system.

Discrete-event system.

In DEVS, external transition function, internal transition function, output function and time advance function are defined on S [START_REF] Zeigler | Theory of Modeling and Simulation[END_REF]:

• δext : Q × X → S with Q = {(s, e)|s ∈ S, 0 ≤ e ≤ ta(s)} • δint : S → S • λ : S → Y • ta : S → R + 0,∞
with S characterized by a set of state variables and their values, i.e. S = (sv, v)|sv ∈ SV, v ∈ Dsv such that SV is the set of state variables and Dsv is the domain of value for state variable sv.

For example, the state space S for a buffer BUF is defined by: SBUF = {(proc status, {f ree, busy}), (queue, {0; n}|n ∈ N)} Or the state space S of a quantized integrator I is defined by:

SI = {ql ∈ R, q ∈ R, q ∈ R, σ ∈ R} .
Often, but this is not mandatory, a DEVS atomic component has a special state variable called phase that can take a value from a set list of literal values.

A initial state is given as a list of assignment. For example, if the state variables queue and proc status have the initial values empty and free, then the initial state is given as queue := 0, status := f ree. The general form of the internal state transition function is given as current state ⇒ next state where current state is expressed as a boolean expression and next state as a list of assignment. For example an internal transition buffer BUF could be:

(queue > 0 && proc status == f ree) ⇒ (queue := queue --, proc status := busy) (1)

The general form of a time advance function is given as current state ⇒ R + 0,∞ where current state is expressed as a boolean expression. For exemple a time advance function for the buffer BUF could be:

(queue > 0 && proc status == f ree) ⇒ 0 (2)
In ProDEVS we assume that an atomic component always hold exactly one state variable called phase that can take many values as necessary from a comprehensive list of literal values. A value for this variable will be represented by a UML state. In the remainder of this paper, for simplicity, we will call a phase one value of state variable phase. Thus, in ProDEVS, time advance function and transition functions are associated to each value of phase, or simpler, to each phase, all which is actually restrictive according to DEVS specification.

However, within ProDEVS, the assignments are actions associated with the transitions and the boolean expressions are guards also associated with transitions. For example, a buffer that would be modeled without phase in DEVS will be modeled in ProDEVS by the state machine in Figure 4 2 .

Figure 4: Atomic component BUF with one phase

This mechanism of actions and guards associated with transitions which are themselves a directed relationship between the phases can represent exactly a DEVS model. Indeed, the internal transition function given by equation ( 1) is encoded by the transition 5 Figure 4. Furthermore the time advance function given by equation ( 2) is encoded by the transition 4. The guard expresses the current state, here, (queue > 0 && proc status == busy). The action contains the assignments on the state variables from the current state, here, queue is unchanged and proc status := f ree. The assignment of the state variable sigma allows setting the time advance function for next state.

In fact there are a multitude of ways to model a buffer always with the same behavior. For example it is possible to allocate the state variable proc status in phases. Thus, BUF becomes as shown Figure 5: The finer grain BUF model would allocate queue and proc status into phases. This gives a model without guards and assignements as shown Figure 6.

Continuous system.

Basically, the QSS method consists in discretizing the space of state variables using a fixed value called the quantum size D. According to this quantum, a variable q can only take values among q ± kD where k is an integer. The solution q(t) of a system described by a differential equation is approximated on a grid in the phase space of the system. The resolution of the phase space grid is D. The time h required to move from one phase space grid point to another on q(t) is approximated and a state change will be informed only at this time.

As shown in [START_REF] Nutaro | Discrete event simulation of continuous systems[END_REF], h may be computed from classical ODE solvers, e.g. for Euler or Runge-Kutta. Consider an ordinary differential equation in the form of q = f (q(t))

We consider the simple Euler integration method

q(t + h) = q(t) + h q(t) (4)
Let the quantum D be defined by

D = |q(t + h) -q(t)| (5)
Then the time required for a change of size D to occur on q(t) is approximatively

h = D | q(t)| if q(t) = 0 ∞ otherwise
This approach can easily be extended to a set of coupled ordinary differential equations in the form of q = f (q(t)), where q is a vector of differential variables. For each ordinary differential equation i of such a system, two variables are necessary. A variable qi which is the position of state variable i on its phase space axis and a variable qli which contains the last grid point occupied by the variable qi. These two variables are necessary because the function fi is now computed at grid points in the discrete phase space of each element of the vector q. A variable qj may have reached a grid point in its discrete phase space while the variable qi has not reached its next grid point yet. Then the time required for the variable qli to be updated becomes

h = D-|q i -ql i | | qi (t)| if q(t) = 0 ∞ otherwise
where |qi -qli| is the distance already traveled along the phase space axis of state variable i.

A DEVS description of the quantized integrator is: δint(ql, q, q, σ) = (qn, qn, f (qn, x),

d |f (qn, x)| ) (6)
δcon(ql, q, q, σ) = (qn, qn, f (qn, x), d |f (qn, x)| ) [START_REF] Nutaro | Adevs (a discrete event system simulator)[END_REF] δext((ql, q, q, σ), e, x) = (ql, q + q * e, f (q, x), d -|q + q * e -ql| |f (q, x)| )

λ(ql, q, q, σ) = qn (9)

ta(ql, q, q, σ) = σ [START_REF] Quesnel | The Virtual Laboratory Environment -An operational framework for multi-modelling, simulation and analysis of complex dynamical systems[END_REF] with qn = ql + d * sgn( q) the next value of the integral and function sgn(v) returns - The atomic component figure 8 generates a triangular wave. The internal transition from phase s0 to phase s3 is used to initialise the component with the given parameters a for amplitude and f for frequency. vdx is the derivative and vx is the signal. From state s3 a new value of vx is sent on the output port s0 every σ = d/abs(vdx) where d is the quantum. If vdx is positive (negative), vx is increased (decreased) by d. When vx reaches a or -a, the derivative is inverted. The transitions which map s3 to s2 must have higher priority than the transitions which map s3 to s3.

1 if v < 0, 0 if v = 0 or 1 if v > 0.

EXAMPLE

This section describes an example that shows simulation results within ProDEVS. The application is the control of a DC motor with pulse width modulation. It is taken from the hybrid example folder in PowerDEVS software and detailed in [START_REF] Cellier | Continuous System Simulation[END_REF]. In this application the power signal of the motor is 3 Character e is a reserved character that may not be used as variable name of a component. It contains the elapsed time since the last transition The whole control and plant systems were built using blocks from ProDEVS library which are atomic components described with ProDEVS State Machines. The motor is represented by a second order model (see Figure 10). A torque step is applied after 3 seconds of simulation. The control system is composed of the following components (see Figure 9):

• The input voltage of the motor switches between +500V and -500V depending on the PWM control law.

• For the PWM law, a triangular waveform of 1KHz frequency and an amplitude of 1.1V is considered. The moment at which the triangular wave crosses the value given by u ref , determines the actual voltage applied to the motor.

• The control is using a proportional law, i.e. u ref is proportional to the error ω ref (t) -ω(t).

• The angular velocity reference signal, ω ref (t) is a ramp signal that increases from 0 to 60 rad/sec in 2 seconds.

The simulation results are shown in Figure 11. A simulation across 5 seconds of simulation time took about 27 seconds on a 1.80GHz running under Ubuntu. The same experiment with PowerDEVS took about 15 seconds. The real time of the simulation can be improved using QSS2 methods (205ms with PowerDEVS). We think we can improve the performance of the tool with a different storage management for variable values. At each step of the simulation The tool performs a type conversion in each step to make the calculations. This cast is expensive. Furthermore we believe we can optimize simulation engine processing such as searching ports connection through the hierarchy.

CONCLUSIONS

This paper has introduce a new event-driven modeling and simulation tool for the simulation of hybrid systems. The particularity of this software lies in its graphical language to define the behavior of the atomic components. This graphical language customizes state machines for DEVS and quantized based numerical methods. ProDEVS proposes an intuitive and user-friendly interface for designing DEVS model without experience in programming. It provides a block diagram interface for coupled components where users can go up and done the hierarchy with a just a double-clic. A library promotes the reuse of models where parameters can be tuned without entering the atomic component. This library can be extended by users thanks to the export feature by a simple drag and drop from the diagram panel to the library panel. ProDEVS provides a model verifier to quickly detect inconsistencies and incompleteness.

We are now developing a formal verification feature of a ProDEVS model. Our approach is based on a transformation to extended TPN (Timed Petri Nets) with data handling called (TTS) Time Transition Systems to generate a finite representation for the accessible states of a ProDEVS model.

Finally we are considering a VHDL coder for hardware implementation of parallel architectures on FPGA to boost the performance and provide rapid prototyping features.

Figure 1 :

 1 Figure 1: ProDEVS editor

Figure 2 :

 2 Figure 2: ProDEVS model structure

Figure 3 :

 3 Figure 3: ProDEVS StateMachine

Figure 5 :

 5 Figure 5: Atomic component BUF with three phases

2

  A dotted arrow represents an internal transition and a full arrow represents an external transition.

Figure 6 :

 6 Figure 6: Atomic component BUF with six phases for a buffer with capacity of 2 (taken from [14] page 465).

Figure 7 :

 7 Figure 7: Quantized integratorA ProDEVS State Machine for this quantized integrator is given Figure73 . It is a generic component that can be taken from the library to construct an hybrid system with only two parameter to define: the quantum d and the initial state value vdx.The atomic component figure8generates a triangular wave. The internal transition from phase s0 to phase s3 is used to initialise the component with the given parameters a for amplitude and f for frequency. vdx is the derivative and vx is the signal. From state s3 a new value of vx is sent on the output port s0 every σ = d/abs(vdx) where d is the quantum. If vdx is positive (negative), vx is increased (decreased) by d. When vx reaches a or -a, the derivative is inverted. The transitions which map s3 to s2 must have higher priority than the transitions which map s3 to s3.

Figure 8 :

 8 Figure 8: Triangular wave

Figure 10 :Figure 11 :

 1011 Figure 10: dc-drive model level 1 (dc-drive-coupled)

The term property or structural feature is used in UML. In this model the class Property simply represents a variable of the component.


P Benammar

A Chauvin

Simon

Marguerite Guiguet
email: marguerite.guiguet@inserm.fr

S Dionou

J Volant

M C Samba

N Benammar

P Chauvin

A Simon

Men from Sub-Saharan Africa Living

Keywords: HIV, Never testing, African migrant, Men, France

INTRODUCTION

Recently, the START study confirmed the benefit of early initiation of antiretroviral therapy for HIV-infected individuals (1), but delayed presentation to care among HIV-infected individuals is still frequent [START_REF] Mocroft | Risk factors and outcomes for late presentation for HIV-positive persons in Europe: results from the Collaboration of Observational HIV Epidemiological Research Europe Study (COHERE)[END_REF]. In the Paris region, there are an estimated 13,000 HIVinfected people who are unaware of their HIV status (out of a population of 11.9 million).

Modeling of France's so-called "hidden HIV epidemic" has suggested that a large proportion of these undiagnosed HIV-infected individuals are non-French heterosexual men [START_REF] Supervie | The undiagnosed HIV epidemic in France and its implications for HIV screening strategies[END_REF].

Migrants have been identified as being at risk for late presentation with awareness being raised of barriers to earlier testing, and specific HIV testing recommendations have been made to address this population in most European countries [START_REF] Blondell | Barriers and facilitators to HIV testing in migrants in high-income countries: a systematic review[END_REF]. In Paris, people living in migrant worker hostels constitute a hard-to-reach, predominantly male (94%) population for which there are few public health statistics, population health surveys or, until now, targeted health-related interventions. Built mainly from the 1950s to the 1970s, the 49 hostels in Paris still house 12,700 people, with 8,600 beds available [START_REF] Apur | Les foyers de travailleurs migrants à Paris : État des lieux en 2010 et inventaires des interventions sociales, sanitaires et culturelles[END_REF]. Most (63%) of the residents are from sub-Saharan Africa. In order to design optimal interventions relevant to this key population, a pilot study was conducted in a few hostels. We did not know how asking about health outside medical context would be received and the main issue was to validate the feasibility of health information sessions organized in living settings. We assessed HIV testing rate among the specific population of men living in migrant worker hostels and investigated factors associated with no lifetime HIV testing.

METHODS

Settings

Health education and prevention sessions with emphasis on testing and treatment for HIV, the hepatitis B virus (HBV), and hepatitis C virus (HCV) were organized in four migrant worker hostels located in the neighborhood with Paris's largest public hospital. A health navigator (S.D.) at this hospital has worked for many years with patients from sub-Saharan Africa. Between June 2012 and September 2014, seven sessions took place in the hostels' meeting rooms on a Friday evening (three evenings at one hostel, two evenings at another hostel, and one evening at each of the other two hostels). During these events, the residents were encouraged to have a confidential, face-to-face consultation with a healthcare worker for a rapid health check-up. The consultation was organized in a private room, with translation help from the health navigator, if necessary. Chronic pathologies, vaccinations, and recourse to HIV testing were reviewed with advices. In addition to age, three demographic variables in relationship with migration history were recorded: country of origin, year of immigration, and health insurance coverage. Health insurance coverage could be used as a proxy for administrative status. Both basic health insurance and the French universal health coverage are inaccessible to illegal immigrants. The state medical aid is dedicated to illegal immigrants after 3 months of residence in France but requires long and somewhat complex administrative procedures.

Ethics statement

The study had been reviewed by the Comité de Protection des Personnes Ile-de-France VI. All the participants were informed of the study and gave their consent.

Statistical methods

Descriptive statistics are shown as medians and interquartile ranges, or n's and percentages, with the comparisons based on the Kruskal-Wallis test for continuous variables or χ 2 tests (or the Fisher exact test when the numbers were small) for categorical variables. Factors associated with never testing for HIV were studied using logistic regression. Statistical analyses were performed with SAS v9.3 software (SAS Institute Inc.).

RESULTS

A total of 550 men had a consultation. Nearly all were from Sub-Saharan Africa (96%), mainly Mali (74%) and Senegal (14%). The median age was 40 years (IQR, 33-51), with no difference between the hostels (Table 1). The median length of residence in France was 12 years (IQR, 3-23), with 29%, 12% and 59% having lived in France for less than 5 years, 5 to 9 years, and 10 years or more, respectively. Nearly half of the participants reported having at least basic health insurance or the French universal health coverage, 22% were covered by the state medical aid, and 25% had no health insurance at all. In all, 63% had never been tested for HIV. This proportion was similar in all but one of the hostels. As shown in Table 1, having no health insurance was the main risk factor for no previous HIV testing (adjusted odds ratio (aOR), 2.4; 95% confidence interval (CI), 1.4 to 4.0).

The men who have lived in France for less than 5 years were twice as likely to report no previous testing than those with at least 10 years of residence (aOR, 1.9; 95% CI, 1.1 to 3.5).

After adjustment, younger age was no longer associated with previous HIV testing, but the men over 50 years were more likely to have never been tested than those aged 30 to 39 years (aOR, 1.9; 95% CI, 1.1 to 3.2).

DISCUSSION

French national policy makes migrants a priority for HIV screening, but our results strengthened our hypothesis of a gap in achieving universal voluntary HIV testing. Only 37% of the migrant men living in worker hostels who participated in health educational sessions in their living settings had been tested for HIV at least once in their lifetime, compared to a proportion of 57% among a representative sample of men living in the Paris area in 2010 [START_REF] Massari | Which adults in the Paris metropolitan area have never been tested for HIV? A 2010 multilevel, cross-sectional, population-based study[END_REF].

Fear of stigma and lack of confidentiality have been reported as preventing HIV testing in migrants [START_REF] Blondell | Barriers and facilitators to HIV testing in migrants in high-income countries: a systematic review[END_REF]. With nearly half of participant likely illegal immigrants as indicated by their lack of common health insurance coverage, this pilot study evidenced a large feasibility of health information sessions and personal health review organized in living settings.

The four hostels were a convenient sample for the pilot study and were all located in close proximity to the hospital that organized health educational sessions. Based on an inventory of the 49 migrant worker hostels in Paris published in 2011 (5), we observed an higher percentage of people from Sub-Saharan Africa in our respondents than reported (96% vs 63%). Unfortunately, due to a large overpopulation in migrant hostels, with 1.6 resident per official resident, no other demographic characteristics could be compared as only official residents were described in the report.

The main limitation of this study was the paucity of sociodemographic data. The men's residency status, from a legal standpoint, was unknown to us, but their length of residence in France may approximate it, since illegal immigrants can expect to become legal after a few years of residence. No data on education level, employment status or socio-occupational categories were collected either. However, living in collective dwellings, such as hostels for immigrants, instead of ordinary households many years after immigrating most likely reflects a less favorable financial situation (as well as the absence of a long-term partnership). The only other information we had was their health insurance status, which correlated, at least partially, with their employment and administrative statuses. Lastly, we did not have any data on their health care utilization. The circumstances surrounding previous HIV testing (e.g., voluntary or during a routine examination) were not known.

The difference in HIV testing according to migration origin is controversial. An increased risk of delayed diagnosis has been observed among migrants (2), and there is an effort to close this gap. In surveys in the general population living in the Paris metropolitan area, 60% of men of sub-Saharan nationality reported having been tested for HIV during their lifetime [START_REF] Lapostolle | Differences in recourse to HIV testing according to migration origin in the Paris metropolitan area in 2010[END_REF][START_REF] Vu | Les populations africaines d'Ile-de-France face au VIH/SIDA : connaissances, attitudes, croyances et comportements[END_REF]. However, these surveys did not include residents of worker hostels or non-Frenchspeaking people. Our results are the first ones in France that show how low the HIV screening rate can be in specific groups of foreigners, with only 37% of migrants living in worker hostels having been tested in their lifetime. This screening rate is of the same magnitude as that (around 30%) reported annually by one of the largest medical nongovernmental organizations in France that serves migrant patients, who visit its network of free clinics. However, this population is much poorer and more excluded than that in our study, with half being undocumented and almost all living in poverty, including a fourth who have no source of income. We could not rule out the possibility that recall bias resulted in an underestimate of the proportion of men in our study who had previously been tested.

However, the participants in all the studies were asked to recall their HIV testing history, so it is unlikely that the large differences in the proportions of first-time testers is due to this potential bias.

Many opportunities for HIV testing exist in France, with a total of 5.2 million HIV tests having been performed in 2013 [START_REF] Cazein | Dépistage de l'infection par le VIH en France, 2003-2013[END_REF]. While there is systematic prenatal HIV testing for women, such testing is proposed less systematically to men during routine medical examinations. A study of newly HIV-diagnosed patients reported that 45% of sub-Saharan migrants who discovered their infection had never been tested, even though nearly all had had medical encounters during the three years preceding the diagnosis [START_REF] Champenois | Missed opportunities for HIV testing in newly-HIV-diagnosed patients, a cross sectional study[END_REF]. Demographic, migratory and behavioral factors have been associated with HIV testing in migrants (4). Intrinsic cultural factors have been advanced for delayed diagnosis while barriers in accessing the health system are more likely at the root of missed opportunities [START_REF] Adedimeji | Increasing HIV testing among African Immigrants in Ireland: Challenges and opportunities[END_REF]. Assimilation status linked to duration of immigration and fluency to communicate in the host country language, as well as access to healthcare has been shown to predict access to HIV prevention [START_REF] Glasman | Disparities in access to HIV prevention among men of mexican descent living in the midwestern United States[END_REF]. In our study, the main risk factor for never having been tested was having no health insurance which could reflect a more general distance to public services. To increase HIV testing opportunities, free and anonymous HIV testing is available at the 13 dedicated public screening centers across Paris.

However, among migrants living in the Paris metropolitan area, the level of knowledge of these dedicated consultations was lower among those without health insurance [START_REF] Vu | Les populations africaines d'Ile-de-France face au VIH/SIDA : connaissances, attitudes, croyances et comportements[END_REF]. Even if no insurance is needed to avail oneself of these centers, the fear of being noticed can prevent migrants with illegal residence status from seeking out preventive healthcare in this city.

In conclusion, despite a broad offer of free HIV testing and regular campaigns promoting such screening, a large proportion of migrant men living in worker hostels still report never having been tested for HIV. To overcome barriers to HIV testing, a recent meta-analysis concluded that alternative approaches such as community-based HIV testing should be offered in addition to standard voluntary testing in healthcare facilities and providerinitiated HIV testing [START_REF] Suthar | Towards universal voluntary HIV testing and counselling : A systematic review and meta-analysis of communitybased approaches[END_REF]. Our results highlight the need to develop such new strategies adapted to specific populations.

ACKNOWLEDGEMENTS.

We gratefully thank all the hostel residents and healthcare staff who participated in this study. Lastly we thank Mark Wikens for his valuable help in revising this article.

COMPLIANCE WITH ETHICAL STANDARDS

Ethical approval

All procedures performed were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study had been reviewed by the Comité de Protection des Personnes Ile-de-France VI.

Informed consent

Informed consent was obtained from all individual participants included in the study.

Table 1 :

 1 Characteristics of 550 men living in four migrant worker hostels in Paris, France, and factors associated with no lifetime HIV testing Basic HI with or without supplementary HI (n=214, 39.6%), French universal health coverage (n=33, 6.1% ), state medical aid (n=120, 22.2% ), or private HI (n=40, 7.4%).

	N	% never tested	Crude OR° (95% CI)	p	Adjusted OR (95% CI)	p

*


Nada Mimouni

Adeline Nazarenko

Sylvie Salotti

Answering Complex Queries on Legal Networks : a Direct and a Structured IR Approaches

Keywords: Semantic information retrieval, Legal collection graphs, Exact answers, Approximate search, Knowledge discovery

This paper highlights the benefit of semantic information retrieval in legal networks. User queries get more complex when they combine constraints on semantic content and intertextual links between documents. Comparing two methods of search in legal collection networks, we present new functionalities of search and browsing. Relying on a structured representation of the collection graph, the first approach allows for approximate answers and knowledge discovery. The second one supports richer semantics and scalability but offers fewer search functionalities. We indicate how those approaches could be combined to get the best of both.

Introduction

Legal practitioners are permanently in the need to search legal collections in order to assess regulatory texts applicable to a specific case. Needs analysis with legal experts and partners of the Légilocal project [START_REF] Amardeilh | The légilocal project: the local law simply shared[END_REF] showed that users express their needs in the form of complex queries that address both the semantic content and intertextual links between documents. For instance, when drafting an order, municipal clerks typically have to identify the legislation to refer to as well as former orders published on the same topic, especially those that have been appealed. They would enter queries like "Which local acts concerning rural roads have been appealed and were canceled by court decision?".

Legal information retrieval (IR) systems widely used by both citizens and practitioners are not able to handle such complex queries. They return documents based on the keyword they contain or are associated to as metadata and not on intertextual grounds. Answers are returned as a list of documents without taking into account the graphs to which those documents belong (esp. references to other texts). To build the context of a given document, a step of exploratory search -starting from one returned result and going through its hypertextual links -is required. In legal collections, where numerous and various types of links exist between legal sources, users get easily lost.

Moreover, legal IR systems operate as logical search systems since a user needs to get all texts related to his query and not only the most relevant ones. Answers are returned when documents match exactly the sent queries. When no exact answer is found or when too few or too many answers are returned, users are not satisfied with the results and must continue to query the system until they get a relevant response.

In an earlier research, we have proposed an approach towards semantic and graphbased search in networks of legal documents [START_REF] Mimouni | Towards graph-based and semantic search in legal information access systems[END_REF] and demonstrated the potentialities that a graph-based semantic IR approach could offer to legal content management systems. In this approach, the intertextual information is taken into account at the querying level to that it is possible to answer complex queries like "What are the legal decisions that cite Article 1382 of the Civil Code?", "I am looking for decisions that were canceled by the Court of Cassation." or "What are the code articles that have been confirmed and which are cited by the municipal orders dealing with rural roads?".

In this work, we propose to extend that logical approach with advanced search and discovery functionalities, by returning approximate answers when no exact one is found, by allowing to restrict or enlarge the answers set when too many or too few are found and by discovering new knowledge not easily detected by hypertextual navigation from the list of results. Needs analysis showed that such functionalities are increasingly expected. For instance, legal practitioners may need to understand what laws cited by a given text are talking about or what articles citing a given law are dealing with.

We implemented graph-based search using two different approaches: a structured and a direct one. The first approach creates a semantic hierarchical and relational structure on the top of the collection thus allowing for advanced search functionalities but it is limited to sub-collections 2 . The second approach can handle larger collections but supports only exact querying.

We present our experimental data on section 2 and our methods of graph-based IR in Section 3. Exact search and browsing functionalities are discussed respectively in Sections 4 and 5. Section 6 introduces related works addressing the problem of modeling and querying graphs of legal collections.

Experimental Data

We test and evaluate our approaches on three collections of documents: [START_REF] Mimouni | Towards graph-based and semantic search in legal information access systems[END_REF].

• The ILO corpus is the largest collection with almost 400 documents collected from the International Labour Office (ILO) 3 . Documents are conventions and recommendations, linked by an implementation relation.

These collections have served us as a primary base to test the proposed approaches. The documents are analyzed to identify their types and extract their structure, they are annotated with semantic descriptors and the intertextual citations are themselves identified and semantically typed. Semantic descriptors annotating documents correspond to concepts extracted from semantic resources dealing with the texts domains. Real scale collections are to be created and documents analyzed by partners of the project.

Needs analysis realized with collaboration of Légilocal legal experts shows that practitioners' spontaneous queries naturally combine content and intertextual criteria. We built a list of queries which are formally different from each other but which covers a wide range of practitioners' queries. We created a set of queries on both ILO and Légilocal (LEGI) corpus along with their relevant answers to test and evaluate our search approaches, the quality of a method being related to the variety of query types that it supports.

• ILO1 : "Which convention implements Recommendation 113 on collective bargaining?" • ILO2 : "Which convention implements the recommendation talking about seafarers occupational accidents?" • ILO3 : "What are the recommendations implemented by conventions dealing with air pollution?" • LEGI1 : "What are the legal decisions that cite Article 1382 of the Civil Code?" • LEGI2 : "What are the articles of the Environment Code that deal with motor vehicles and rural roads?" • LEGI3 : "I am looking for the decision that is the subject of the judgment A of the Court of Cassation." • LEGI4 : "I am looking for decisions that were canceled by the Court of Cassation." • LEGI5 : "I wonder if this judgment of the Court of Appeal was itself the subject of an appeal." • LEGI6 : "I would like to see municipal orders regarding rural roads that have been appealed and were canceled by decision of jurisprudence." • LEGI7 : What are the code articles which are cited by the municipal orders dealing with rural roads that have been confirmed? • LEGI8 : "What are the decisions prior to decision D?" • LEGI9 : "I wonder if the texts referred by municipal orders dealing with rural roads are also cited by those concerning the motor vehicles." • LEGI10 : "What are municipal orders that have been the subject of two appeals?" • LEGI11 : "I wonder if the texts referred by the municipal order 97-17 of Champigné have been modified, and if so, what are the new versions of these texts and the source texts of the amendment."

Modeling Legal Collections as graphs

The document collection is modeled as a graph of documents, which can be exploited directly using semantic technologies (direct approach) or which can be further structured into a hierarchical and relational conceptual structure, that itself supports search and browsing (structured approach). The direct approach models the collection as an RDF graph where the objects (nodes) are document and descriptor identifiers and where properties (edges) are intertextual links (between documents) or annotations (between documents and descriptors). An OWL document ontology controls the semantics of the RDF graph. It provides for fine-grained descriptions of legal collections, taking any type of features into account: semantic metadata, document structure (sections, paragraphs, etc.), document types (law, decree, etc.) and intertextual relationships. User queries are expressed in SPARQL and are directly matched against the RDF graph.

Relying on FCA/RCA, the structured approach [START_REF] Mimouni | A conceptual approach for relational IR: application to legal collections[END_REF] models the collection as a set of interlinked hierarchical structures which organizes documents into classes on the basis of their semantic content and their interrelationships. Compared with the direct approach, the initial modeling step is simpler (the collection is modeled as a set of objets/attributes structures) but there is an additional structuring phase. Similarly querying the structure is straightforward, as any answer is already pre-computed as a class of documents, but the structure can be also exploited for browsing.

Figure 1 compares these approaches. We can notice that modeling and encoding the collection are more sophisticated with the direct approach (ontological model and RDF triples) than with the structured approach (binary tables). This allows the direct approach to express more complex query graphs. Reversely, the structured approach has the advantage of structuring the collection.

In information retrieval, structuring a document collection as a lattice pre-computes the answers to all queries that are satisfiable on that collection. In addition, browsing the structure provides approximate answers as it allows for generalizing or restricting queries. In the direct approach, answers are calculated on the fly when the SPARQL query is sent to the system. This approach is more flexible (when the model of the collection or the collection itself evolves), is not limited by the collection size and allows to express richer queries. On the contrary, the structured approach performs well on small collections or local perspectives over large collections and allows browsing and visualizing, but it cannot answer complex queries like LEGI11. We have developed a prototype to visualize the answer graphs from the structured approach displaying the objects of the answer along with all their attributes even if they are not mentioned in the initial query.

The choice to use one or the other approach depends tightly on the application requirements (user interfaces, number of documents, granularity of document description or the evolution of the collection).

Exact Search in Collection Graphs

As presented in [START_REF] Mimouni | Towards graph-based and semantic search in legal information access systems[END_REF], document collections and queries are formalized as graphs. The proposed approaches enable answering different types of elementary and complex queries. We tested 14 types having or not a focus and with or without constraints. For instance, Figure 2 shows two examples of query graphs : q a corresponds to an elementary query and q b to a relational one. The query q a looks for documents of type t 2 annotated with semantic attributes s 4 and s 5 . The query q b looks for documents of type t 1 annotated with s 1 having the link r 1 with documents of type t 2 annotated with s 4 .

? ?

t1 t2 s1 s4 r1 ? t2 s4 s5 q a q b q a = Type(x,t 2 ) ∧ Att(x, s 4 ) ∧ Att(x, s 5 ) q b = (x, y) : Type(x,t 1 ) ∧ Att(x, s 1 ) ∧ Type(y,t 2 ) ∧ Att(y, s 4 ) ∧ Rel(x, r 1 , y)

Figure 2. Examples of query graphs: q a represents an elementary query and q b a relational query, which is represented with a focus in the formula.

On exact search and for elementary as well as for relational queries, both approaches perform well. All the selected queries are properly answered, with a list of documents or document graphs that exactly match the query or no answer if the query is not satisfiable. We show below some examples of queries, testing the direct and the structured approaches on our three test corpus : Noise, ILO and Légilocal corpus.

Structured Approach

Once structured as a family of relational lattices, the collection can be exploited to answer elementary and relational queries. We tested this approach on both Noise and ILO corpus.

Let's consider an example of a relational query on the Noise corpus : "Which orders talk about sound level (nvs) and make reference (rf) to decrees about noisy activities (ab)?". Figure 3 shows the structure built for the collection. The query is modeled as an attributed graph, which nodes are virtual objects (variables) created for each unidentified document (Q o for orders and Q d for decrees), which attributes are semantic descriptors (nvs for node Q o and ab for Q d ) and the link is the intertextual relation (r f ). An iterative search process calculates relevant answers [START_REF] Mimouni | A conceptual approach for relational IR: application to legal collections[END_REF], it instantiates Q o and Q d respec-tively by documents AS (Strasbourg order) and O45 (ordinance 45). It returns the following answer graph (the query has no focus, so answers are presented as graphs): G a = Type(AS, order) ∧ Att(AS, nvs) ∧ Type(O45, decree) ∧ Att(O45, ab) ∧ Rel(AS, r f , 045). We tested the approach on the ILO corpus: all the collected queries returned the expected relevant answers. We detail hereafter two examples: ILO1 and ILO2.

Direct Approach

As the direct approach allows to model the collection in a more fine-grained manner and is more naturally prepared to process large volume of data, it was more interesting to test it on the Légilocal collection. Nevertheless, we have tested it on the ILO corpus in the aim of comparing the approaches.

In the ILO2 query, we search for objects of the class Convention having the relation implement with Recommandation objects described by semantic concepts occupational accidents and seafarer. The query is translated in the fol- ?conv ilo:implement ?recom .

5

}

The expressivity of the query langage enables composing more complex queries dealing with versions and fragments of documents, or with relations with more than two arguments. For instance, the query LEGI11 is translated as follows:

1 SELECT ?text ?newversion ?oldversion ?source

Exploration Facilities

When a query doesn't have any exact answer, it is interesting to return an approximate answer to the user instead of an empty set of results. Also, when the user gets too few or too many answers to his query, he/she should be able to broaden or narrow his/her search by removing or adding constraints to the initial query.

This could be done by exploring the collection space and selecting documents that are more or less close to the position of the (possibly empty) exact answer variable, i.e. documents with less or more semantic or relational properties. The exploration process could be automated and remain transparent to the user (approximate results are calculated by the system and returned), or manual with suitable user interfaces to visualize the data space and navigate within it.

For the time being, this functionality is provided by the structured approach thanks to the structure it builds on the top of the collection, and not by the semantic approach [START_REF] Mimouni | A conceptual approach for relational IR: application to legal collections[END_REF].

Structuring the collection has the advantage of pre-computing the answers to all the satisfiable queries, either elementary or relational, which allows to find approximate answers to users' queries and discover new knowledge, relying on the strategy of [START_REF] Wray | Exploring the information space of cultural collections using formal concept analysis[END_REF] without extra calculation. Some exploration facilities are given in Figure 6.

Such lattice navigation functionalities cannot be implemented on large collections, but is a track that we explore in the idea of simulating the relevant browsing strategies using SPARQL. We can actually propose some navigation scenarios, in the form of automatically generated sequences of SPARQL queries that would simulate the desired navigation strategies. For instance, the following method corresponds to a query relaxation procedure:

1. Starting from the initial query, relax one or more constraints (RDF triples) in the query graph. 2. Instantiate the relaxed SPARQL queries on the graphs of the collection. 3. Propose the obtained results to the user as an alternative or complement to the exact answer.

Approximate Search: examples

Noise Corpus Let's consider the example presented above (Section 4.1) on the noise corpus. It is possible to explore the order lattice by relaxing a semantic constraint or by dropping a relational constraint ending up with an elementary query (as described in Figure 6). For instance, on the structure of Figure 3, if we relax a semantic constraint by removing the descriptor nvs, we search for orders making reference to laws and decrees about noisy activities (ab), which are grouped in concept c13. A relaxed answer-graph G r is given by the document AP linked to the document D95 : G r = Type(AP, order) ∧ Type(D95, decree) ∧ Att(D95, ab) ∧ Rel(AP, r f , D95).

ILO Corpus On Figure 5, the right side of the graph contains, in addition to the exact answer, several approximate answers that have only one of the query attributes : seafarer for R107, R138 and occupational accidents for R164 and R171. They This can be simulated by the following sequence of relaxed SPARQL queries:

Knowledge Discovery: examples

On Figure 5, the left side of the graph represents the convention which implements the recommandation R142. The visualisation module presents all the attributes (semantic descriptors) of the convention C164 on the result graph. This enables the user to extend the search, looking for conventions that talk about similar topics, i.e. which are annotated with a subset of its attributes. This corresponds to the function of search by example [START_REF] Wray | Exploring the information space of cultural collections using formal concept analysis[END_REF] where the user has initially a sample, one document or a set of documents, and looks for similar ones (typically the case of a municipal clerk having to draft a local act and who starts by looking for similar ones in neighboring municipalities). We can at this stage (without navigation interface) present directly to the user the similar objects, by performing a simple search (elementary query with the attributes of C164) on the initial conceptual structure of conventions.

In addition, visualizing the conceptual structure built on the top of a given collection, allows for discovering clusters of objects that share the same properties (semantic or relational) without having to formulate queries. These clusters have served as a basis for the design and restructuring of domain ontologies [START_REF] Hacene | Supporting ontology design through large-scale fcabased ontology restructuring[END_REF].

Related Work

Many works have studied the intertextuality between legal sources and acknowledged that network analysis is a powerful way to model legal collections [START_REF] Fowler | Network analysis and the law : Measuring the legal importance of precedents at the u.s. supreme court[END_REF][START_REF] Romain | A network approach to the french system of legal codes-part i: Analysis of a dense network[END_REF][START_REF] Winkels | Survival of the fittest: Network analysis of dutch supreme court cases[END_REF]. However, as in citation and social network analysis [START_REF] Rubin | Foundations of Library and Information Science[END_REF], focus has been put on the network level to identify the most strongly connected sub-collections or the most influential law sources. Less attention has been paid to the detailed analysis of intertextuality and the semantics of intertextual links.

In major legal access systems, intertextuality is presented as hypertextual links between texts or as a metadata that could be queried (e.g. Legifrance 4 ). New approaches [START_REF] Winkels | Creating context networks in dutch legislation[END_REF][START_REF] Gultemen | Graph-based linking and visualization for legislation documents (glvd)[END_REF] try to enrich legal access systems with contextual networks based on link analysis methods. Their proposals consist mainly on the exploration and visualization of the citations network. They do not take intertextuality into account at the query level as we propose to do in exact and approximate search.

Conclusion

This paper shows that legal practitioners' complex queries could be handled through modeling and querying legal collections as semantic networks. Network nodes (documents) are associated with semantic attributes and connected to each other by various types of semantic links. Queries are themselves modeled as graphs, which are matched against the collection graph so as to return exact or approximate answers to the user. We proposed two methods -direct vs. structured methods -, which have different advantages and drawbacks. Using one model or the other depends tightly on the application: the granularity of the description required for the collection, the complexity of queries, the size of the collection, the relevance of restricted search perspective. Both proposed approaches are logical ones, they are not evaluated in terms of precision and recall. Most important criteria are the expressivity of the query langage (which types of queries could be answered) and performance (collection size, processing time).

Future steps include the implementation of exploration scenarios as sequences of SPARQL queries and the design of adequate interfaces to help users create complex relational queries and analyze the returned results.

Figure 1 .

 1 Figure1. Structured vs. direct approaches. The first approach (top) relies on FCA/RCA to build a semantic structure over the graph of documents. Queries are answered based on that structure. The second approach (bottom) exploits semantic technologies (OWL/RDF/SPARQL) to model the collection as a graph. Query answers are directly extracted from that graph. Compared with the structured approach, the direct one supports richer semantic graphs but offers no exploration facility.

Figure 3 .

 3 Figure 3. Exact and approximate search in a relational lattice family.

ILO1:

  this query contains an identified document R113 (recommandation 113), we create the variable QueryConv corresponding to the searched one. We locate R113, check that it is actually associated with the attribute collective bargaining and search for the related convention. The query graph is: G q = Type(QueryConv, convention)∧Type(R113, recommandation)∧Att(R113, collective bargaining)∧ Rel(QueryConv, impl, R113). The search returns convention C144. The answer graph is depicted on Figure 4, with the dashed line surrounding the exact answer. ILO2 : the query graph contains two variables, two attributes and one relation : G q = Type(QueryConv, convention)∧Type(QueryRec, recommandation)∧Att(QueryRec, occupational accidents)∧ Att(QueryRec, sea f arers) ∧ Rel(QueryConv, impl, QueryRec). The search process instantiates variables QueryConv and QueryRec respectively with documents C164 and R142. The visualisation of the answer graph is given in Figure 5. The central part of the graph (surrounded by the dashed line) corresponds to the exact answer having the attributes of the query.

Figure 4 .

 4 Figure 4. Answer graph for query ILO1.

Figure 5 . 1 SELECT ?conv ?recom 2 WHERE { 3 ?

 5123 Figure 5. Exact and approximate answer graphs for query ILO2.

4

 4

2 WHERE { 3 ?

 23 visa rdf:type lido:VisaCitation .

4 ?

 4 visa lido:citationSource :ArreteChampigne97-17 .

5 ?

 5 visa lido:citationTarget ?text .

6 ?

 6 modification rdf:type lido:Modification .

7 ?

 7 modification lido:referenceTarget ?oldversion .

8 ?

 8 modification lido:referenceSource ?source .

9 ?

 9 modification metalex:result ?newversion .

10 ?

 10 text metalex:realizedBy ?oldversion .

11 } 5 .

 115 Browsing for Approximate Search and Knowledge Discovery

Figure 6 .

 6 Figure 6. Exploring facilities offered by the conceptual structure : query broadening and restricting for an elementary query (diagram 1) or a relational query (diagram 2), suppressing a relational attribute (diagrams 3), adding a relational attribute (diagram 4) are obtained by upward navigating the conceptual structure which accounts for query generalization.This can be simulated by the following sequence of relaxed SPARQL queries:

•

  The Noise corpus was used as an illustrative example in Légilocal. It is com-

	posed of 10 documents annotated with 10 attributes related to noise and a single
	reference relation. Documents are of different types: local acts (orders) making
	reference to legislative texts (decree, law, code, ordinance).
	• The Légilocal corpus is a richer collection extracted from the Légilocal base
	for demonstration purposes: it is composed of 25 documents and 30 legal arti-
	cles, various types of documents are represented (local decisions, legislative texts,
	judgements and editorial documents) -sometimes with different versions of the
	same document -, as well as various types of links (application, composition,
	decision either confirmation or cancelation, modification)

This work has been partially funded by French Single Inter-Ministry Fund (FUI-9, 2010-2013 and is supported by the French National Research Agency (ANR-10-LABX-0083) in the context of the Labex EFL

Corresponding Author: N. Mimouni; E-mail: nada.mimouni@lipn.univ-paris13.fr

The target scale is a few hundred documents, but we must further optimize the code for it (solutions exist, it is an implementation issue).

www.ilo.org

http://www.legifrance.gouv.fr/


Chaofeng Zhu

Xianghua Zhang
email: xiang-hua.zhang@univ-rennes1.fr

Hongli Ma

Sb/Mn co-doped oxyfluoride silicate glasses for potential applications in photosynthesis

Keywords: Luminescence, Glasses, Photosynthesis, Energy transfer

A series of Sb/Mn co-doped oxyfluoride silicate glasses were prepared via the melt-quenching method to explore red luminescent materials for potential applications in photosynthesis of green plants, and these glasses are investigated by means of luminescence decay curves, absorption, emission, and excitation spectra. We find that the as-prepared glasses are transparent in the visible region and can emit strong red light under ultraviolet, purple, and green light excitations. Furthermore, energy transfer from Sb 3+ to Mn 2+ ions occurs in Sb/Mn co-doped glasses. The results demonstrate that the as-prepared Sb/Mn co-doped oxyfluoride silicate glasses may serve as a potential candidate for developing glass greenhouse, which can enhance the utilization of solar energy for the photosynthesis of the green plants.

Introduction

Photosynthesis is a process to convert light energy, normally from the sun, into chemical energy that can be later released to fuel the plants' activities. In the solar spectrum, the energy percentage of the ultraviolet (UV), visible, and infrared lights is about 7%, 50%, and 43%, respectively [1]. Generally, the green plants (chlorophylls) absorb more red light than other wavelengths for the photosynthesis. In addition, the green light can hardly be absorbed by the green plants for photosynthesis and the UV light is harmful to the growth of the plants. The lights absorbed by green plants for photosynthesis are only a very small portion in the whole solar spectrum [2,3]. Thus, to convert the UV and green lights into red light is a very promising and valuable work for the growth of the plants. At present, most greenhouses are made of the plastic materials, which have some drawbacks such as short lifetime, poor transparency, and instability. In recent years, the glass greenhouses are attracting more and more research attention due to the superior light transmittance, low production cost, and long service life compared to plastic greenhouses [2,3]. In particular, luminescent glasses can be used for the realization of solar spectrum modification, e.g., to convert the ultraviolet and green lights into the red light for photosynthesis. In other words, luminescent glasses are strongly expected to be an interesting and excellent alternative approach to fabricate greenhouses compared to traditional plastic materials.

The oxyfluoride silicate glasses possess excellent chemical stability, good mechanical strength, high transparency for visible light, as well as low phonon energy environment for luminescence ions [4,5]. Sb and Mn are regarded as promising low-cost and environmental-friendly elements compared to rare earth. Additionally, Sb 2 O 3 can also be used as a fining agent in glass melting. Based on above considerations, herein we fabricated a series of Sb-, and Mn-doped oxyfluoride silicate glasses for greenhouse application and the optical properties of the glasses are investigated. Although some preliminary works on the luminescent glasses or glass-ceramics for greenhouses have been reported [2,3], the Sb/Mn co-doped oxyfluoride silicate glasses have not been considered for photosynthesis applications yet.

Experimental

All the investigated glasses have the glass matrix (GM) composition of 45SiO 2 -15Al 2 O 3 -10ZnO-20CaF 2 -10CaO (in mol.%). Samples GMS2 (doped with 2 mol.% Sb), GMM4 (doped with 4 mol.% Mn), GMS2M2 (co-doped with 2 mol.% Sb and 2 mol.% Mn), GMS2M4 (co-doped with 2 mol.% Sb and 4 mol.% Mn), and GMS2M6 (co-doped with 2 mol.% Sb and 6 mol.% Mn) were prepared by melt-quenching method. The chemicals SiO 2 , Al 2 O 3 , ZnO, CaF 2 , CaCO 3 , Sb 2 O 3 , and MnCO 3 were used as raw materials and they are analytical grade reagents. The starting materials calculated based on the compositions of the as-designed samples were taken in an agate mortar and grounded well to obtain homogenous mixtures. The homogenous batches were put into corundum crucibles and melted in an electrical furnace at 1460°C for 1 hour in air atmosphere.

After melting, the glass melts were quickly casted into a preheated stainless steel mould for quenching.

To relieve the internal stress of these glasses, which is induced during the quenching process, the obtained glasses were annealed at 450 °C for 2 hours in air atmosphere, followed by natural cooling.

The energy dispersive spectroscopy (EDS) was measured using an EDS detector attached on the Scanning Electron Microscope (SEM, Hitachi S-4800, Japan) to analyze the compositions of the glasses. The absorption spectra were recorded using a Lambda 1050 UV/VIS/NIR spectrometer (PerkinElmer, USA). The excitation and emission spectra were measured using an FLS920 fluorescence spectrometer (Edinburgh Instruments Ltd., UK) using a 450 W Xe lamp (Xe900) as light source. The fluorescence decay curves were measured by using a standard storage digital oscilloscope and by exciting the samples at 266 nm with a frequency-quadrupled Nd:YAG laser providing 6 ns laser pulses. All the measurements were performed at room temperature with the same experimental parameters.

Results and Discussion

Since some raw materials, e.g., CaF 2 and ZnO, can be volatile during the melting process, the final compositions of as-prepared glasses may deviate the nominal ones as designed. All the mentioned glasses in this paper have the identical glass matrix compositions. As an example, we carried out the composition analysis of sample GMS2 using EDS technique, as shown in Fig. 1. It can be seen that the content of F, Zn, and Sb decreased compared to the nominal compositions, indicating that these three elements were partially lost during preparing these glasses by melt quenching method. waste for the photosynthesis of the green plants. Thus, it is very interesting to transfer the UV and green lights into the red light, which is helpful for photosynthesis. We can see in Fig. 3 that the as-prepared glasses can effectively convert the UV and blue-green lights into red emission, implying that these materials can improve the utilization of the solar energy for photosynthesis of green plants.

Fig. 4 shows the emission spectra of the Mn 2+ single doped and Sb 3+ /Mn 2+ co-doped glasses excited at 415 nm. The broad emission bands come from the electronic transitions of 4 T 1g → 6 A 1g of Mn 2+ ions [10]. With the increasing of the Mn 2+ ion concentration, the emission intensity increases firstly and then decreases, which could be due to the luminescence concentration quenching of Mn 2+ caused by the cross-relaxation between Mn 2+ ions. The red-shift of the emission peak of Mn 2+ is also observed with increasing Mn 2+ content. This phenomenon could be induced by the following aspect: increasing Mn 2+ content results in the reduction of Mn 2+ -Mn 2+ distance, which leads to the enhancement of the interaction of Mn 2+ -Mn 2+ . That is to say, the ligand field strength surrounding Mn 2+ is enhanced, making the excited state of Mn 2+ energetically closer to its ground state and finally gives a longer wavelength emission [11]. Comparing the emission spectra of samples GMM4 and GMS2M4, it is found that emission blue-shift occurs with the introduction of Sb 3+ ions. The emission of transition metal Mn 2+ ion corresponds to d-d transition, and its emission is sensitive to the surrounding environment [12]. The introduction of Sb 3+ may lead to the variation of the local environment around Mn 2+ ions, which might be the reason for emission blue-shift of sample GMS2M4.

To further investigate the conversion of UV to red light via the as-prepared glasses, the emission spectra for samples GMS2, GMS2M2, GMS2M4, and GMS2M6 excited at 266 nm are illustrated in Fig. 5. It can be seen that these glasses present a broad emission band ranging from 350 to 505 nm, which can be assigned to the electronic transition of 3 P 1 → 1 S 0 of Sb 3+ . With the increase of the Mn 2+ concentration, the intensity of the 350-505 nm emission from Sb 3+ decrease, whereas the intensity of the 550-800 nm emission from Mn 2+ increases firstly and then decreases (luminescence concentration quenching of Mn 2+ ). This phenomenon indicates the energy transfer (ET) from Sb 3+ to Mn 2+ occurs. The ET from Sb 3+ to Mn 2+ can enhance the red emission intensity and cause the variety of emission color of the glasses.

Excitation spectra (λem = 661 nm) of samples GMM4 and GMS2M4 and emission spectra of samples GMS2 ( λex = 266 nm) and GMM4 (λex = 415 nm) are presented in Fig. 6. The emission spectrum of sample GMS2 (λex = 266 nm) gives a broader emission band related to the 3 P 1 1 S 0 transition of Sb 3+ . It should be noted that there is a significant overlap between the emission spectrum of Sb 3+ doped glass GMS2 and the excitation spectrum of Mn 2+ doped glass GMM4 in the range of 350 to 640 nm. Therefore, it is expected that the ET process can efficiently occur from Sb 3+ to Mn 2+ . In order to further clarify this, the excitation spectrum of sample GMS2M4 monitored at 661 nm is also included in Fig. 6. A broad excitation band at ~ 261 nm exists for sample GMS2M4 compared to GMM4, which further confirms the ET process of Sb 3+ to Mn 2+ . It is known that the transition metal Mn 2+ can provide a broad emission band corresponding to d-d transition. Since the absorption and emission of Mn 2+ are weak due to the spin-and parity-forbidden d-d transition of Mn 2+ [13], herein, we employ Sb 3+ as efficient sensitizing ions to enhance the luminescence of Mn 2+ by means of ET.

To further evaluate the energy transfer from Sb 3+ to Mn 2+ in the as-prepared glasses, the decay curves of emission of Sb 3+ were measured with excitation at 266 nm and monitoring at 408 nm for samples GMS2, GMS2M2, GMS2M4 and GMS2M6, as shown in Fig. 7. The decay processes of these four samples are characterized by average lifetime , which can be derived from:

0 0 ) ( / ) (

Conclusions

In summary, Sb-and Mn-doped oxyfluoride silicate glasses were synthesized by melt quenching method. We find that the as-prepared glasses demonstrate high transparency in the visible light range and intense absorption in the ultraviolet region. We use the Sb/Mn co-doped glasses as light converter to successfully transfer the ultraviolet and green lights into the red light, which could be efficiently absorbed by green plants for their photosynthesis.

Additionally, energy transfer from Sb 3+ to Mn 2+ occurs in the glasses. Our investigation shows that Sb/Mn co-doped glasses may be used for the fabrication of greenhouses in the future for enhancing the photosynthesis of the green plants.

Fig. 2

 2 Fig.2shows the absorption spectra of samples GMS2, GMS2M2, GMS2M4, and

Fig. 1 .

 1 Fig. 1. EDS spectrum of sample GMS2. The inset shows the comparison of the nominal and

Fig. 2 .

 2 Fig. 2. Absorption spectra of the glasses. The inset shows the enlarged absorption spectra in

Fig. 3 .

 3 Fig. 3. Excitation and emission spectra of sample GMS2M4. The electronic transitions,

Fig. 4 .

 4 Fig. 4. Emission spectra of glasses excited at 415 nm.

Fig. 5 .

 5 Fig. 5. Emission spectra of glasses under 266 nm excitation.

Fig. 6 .

 6 Fig. 6. Excitation spectra of samples GMM4 and GMS2M4 monitored at 661 nm; Emission

Fig. 7 .

 7 Fig. 7. Luminescence decay curves of samples GMS2, GMS2M2, GMS2M4 and GMS2M6,

Fig. 1 .

 1 Fig. 1. EDS spectrum of sample GMS2. The inset shows the comparison of the nominal and

Fig. 2 .

 2 Fig. 2. Absorption spectra of the glasses. The inset shows the enlarged absorption spectra in

Fig. 3 .

 3 Fig. 3. Excitation and emission spectra of sample GMS2M4. The electronic transitions,

Fig. 4 .

 4 Fig. 4. Emission spectra of glasses excited at 415 nm.

Fig. 5 .

 5 Fig. 5. Emission spectra of glasses under 266 nm excitation.

Fig. 6 .

 6 Fig. 6. Excitation spectra of samples GMM4 and GMS2M4 monitored at 661 nm; Emission spectra of samples GMS2 and GMM4 excited at 266 and 415 nm, respectively.

Fig. 7 .

 7 Fig. 7. Luminescence decay curves of samples GMS2, GMS2M2, GMS2M4 and GMS2M6,

Acknowledgements

This work is financially supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars (Grant No.20110403) of Jinan, China. The authors also gratefully thank Shandong Provincial Education Association for International Exchanges for offering a visiting scholar fellowship to Chaofeng Zhu to perform this work in Université de Rennes 1.

We express our sincere thanks to Richard Moncorgé (Université de Caen, France) for luminescence decay curve measurements and helpful discussions.