# Mixed Linear Complementarity Problems (MLCP)¶

## Problem statement¶

Find $$(z,w)$$ such that:

$\begin{split}\left\{ \begin{array}{l} M \ z + q = w \\ w_1=0 \\ 0 \le w_{2} \perp v \ge 0 \end{array} \right. \text{ with } z= \left[ \begin{array}{c} u\\ v\\ \end{array} \right] \text{ and } w= \left[ \begin{array}{c} w_{1}\\ w_{2}\\ \end{array} \right]\end{split}$

$$u, w_{1}$$ are vectors of size n.

$$v, w_{2}$$ are vectors of size m.

Another storage is also possible for the MLCP problem:

Try $$(u,v,w)$$ such that:

$$\left\lbrace \begin{array}{l} A u + Cv +a =0\\ D u + Bv +b = w \\ 0 \le v \perp w \ge 0\\ \end{array} \right.$$

where A is an ( $$n \times n$$ ) matrix, B is an ( $$m \times m$$ ) matrix, C is an ( $$n \times m$$ ) matrix,

D is an ( $$m \times n$$ ) matrix, a and u is an ( $$n$$ ) vectors b,v and w is an ( $$m$$ ) vectors.

## Implementation in numerics¶

Structure to define the problem: MixedLinearComplementarityProblem.

The generic driver for all MLCPs is mlcp_driver().

Solvers list MLCP_SOLVER

MLCP solvers must be initialized before any call of the driver. Here is the standard sequence of calls:

1. Initialize the solver with mlcp_driver_init().

2. Solve the problemt with mlcp_driver().

3. Reset the solver with mlcp_driver_reset().

## Error computation¶

The criterion is based on :

$error = \frac{1}{\|q\| }\sum_{i} [ (z[i]*(Mz+q)[i])_{+} + (z[i])_{-} + (Mz+q)[i])_{-} ]$

with $$x_{+} = max(0,x)$$ and $$x_{-} = max(0,-x)$$.

## MLCP available solvers¶

### Projected Gauss-Seidel (SICONOS_MLCP_PGS)¶

Projected Gauss-Seidel, a basic Projected Gauss-Seidel solver for MLCP.

driver: mlcp_pgs()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 1000

• iparam[SICONOS_IPARAM_MLCP_PGS_EXPLICIT] = 0, 1 for implicit.

• dparam[SICONOS_DPARAM_TOL] = 1e-6

### Projected Gauss-Seidel, SBM (SICONOS_MLCP_PGS_SBM)¶

Gauss-Seidel with sparse-block storage.

driver: mlcp_pgs_sbm()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 1000

• dparam[SICONOS_DPARAM_TOL] = 1e-6

internal solver : :enumerate:SICONOS_LCP_PGS.

out * iparam[SICONOS_IPARAM_MLCP_PGS_SUM_ITER], sum of local number of iterations (output from local_driver) * dparam[SICONOS_DPARAM_MLCP_PGS_SUM_ERRORS] sum of local errors (output from local_driver)

### Regularized Projected Gauss-Seidel (SICONOS_MLCP_RPGS)¶

Regularized Projected Gauss-Seidel, solver for MLCP, able to handle with matrices with null diagonal terms

driver: mlcp_rpgs()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-6

• dparam[SICONOS_DPARAM_MLCP_RHO] = 0.5

### PSOR (SICONOS_MLCP_PSOR)¶

Projected Succesive over relaxation solver.

driver: mlcp_psor()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 1000

• dparam[SICONOS_DPARAM_TOL] = 1e-6

• dparam[SICONOS_DPARAM_MLCP_OMEGA] = 2

### RPSOR (SICONOS_MLCP_RPSOR)¶

Regularized projected successive overrelaxation method.

driver: mlcp_rpsor()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 1000

• dparam[SICONOS_DPARAM_TOL] = 1e-6

• dparam[SICONOS_DPARAM_MLCP_OMEGA] = 2

• dparam[SICONOS_DPARAM_MLCP_RHO] = 0.5

### PATH (Ferris) solver (SICONOS_MLCP_PATH)¶

Path (Ferris) Solver.

Works only if Siconos has been built with path support (if PathFerris or PathVI has been found, see :ref:siconos_install_guide)

driver: mlcp_path()

parameters * dparam[SICONOS_DPARAM_TOL] = 1e-12

### Enumerative solver (SICONOS_MLCP_ENUM)¶

Brute-force method which tries every possible solution.

driver: mlcp_enum()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• iparam[SICONOS_IPARAM_MLCP_ENUM_USE_DGELS] = 0 (0 : use dgesv, 1: use dgels)

• dparam[SICONOS_DPARAM_TOL] = 1e-12

### PATH + enum solver (SICONOS_MLCP_PATH_ENUM)¶

First try with Path (Ferris) Solver then use enum if the solver failed.

Works only if Siconos has been built with path support (if PathFerris or PathVI has been found, see :ref:siconos_install_guide)

driver: mlcp_path_enum()

parameters : same as SICONOS_MLCP_ENUM.

### Direct + enum solver (SICONOS_MLCP_DIRECT_ENUM)¶

First try direct method and then use enum if the solver failed. driver: mlcp_direct_enum()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

• dparam[SICONOS_IPARAM_MLCP_ENUM_USE_DGELS] = 0;

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_POS] = 1e-12: A positive value, tolerance to consider that complementarity holds.

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_NEG] = 1e-12: A positive value, tolerance to consider that a var is negative.

• iparam[SICONOS_IPARAM_MLCP_NUMBER_OF_CONFIGURATIONS] = 3 : Number of registered configurations.

• iparam[SICONOS_IPARAM_MLCP_UPDATE_REQUIRED] = 0;

• iparam (out): Number of case the direct solved failed.

### Simplex solver (SICONOS_MLCP_SIMPLEX)¶

driver: mlcp_simplex()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

### Direct/Simplex solver (SICONOS_MLCP_DIRECT_SIMPLEX)¶

Try direct method and switch to simplex if it fails.

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_POS] = 1e-12: A positive value, tolerance to consider that complementarity holds.

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_NEG] = 1e-12: A positive value, tolerance to consider that a var is negative.

• iparam[SICONOS_IPARAM_MLCP_NUMBER_OF_CONFIGURATIONS] = 3;

• iparam[SICONOS_IPARAM_MLCP_UPDATE_REQUIRED] = 0;

• iparam (out): Number of case the direct solved failed.

### Direct/Path solver (SICONOS_MLCP_DIRECT_PATH)¶

Try direct method and switch to Path if it fails.

driver: mlcp_direct_path()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_POS] = 1e-12: A positive value, tolerance to consider that complementarity holds.

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_NEG] = 1e-12: A positive value, tolerance to consider that a var is negative.

• iparam[SICONOS_IPARAM_MLCP_NUMBER_OF_CONFIGURATIONS] = 3;

• iparam[SICONOS_IPARAM_MLCP_UPDATE_REQUIRED] = 0;

• iparam (out): Number of case the direct solved failed.

### Direct/Path/enum solver (SICONOS_MLCP_DIRECT_PATH_ENUM)¶

Try direct then switch to PATH and finish with enum.

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_POS] = 1e-12: A positive value, tolerance to consider that complementarity holds.

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_NEG] = 1e-12: A positive value, tolerance to consider that a var is negative.

• iparam[SICONOS_IPARAM_MLCP_NUMBER_OF_CONFIGURATIONS] = 3;

• iparam[SICONOS_IPARAM_MLCP_UPDATE_REQUIRED] = 0;

• iparam[SICONOS_IPARAM_MLCP_ENUM_USE_DGELS] = 0 (0 : use dgesv, 1: use dgels)

• iparam (out): Number of case the direct solved failed.

### Nonsmooth Newton solver, Fisher-Burmeister (SICONOS_MLCP_FB)¶

driver: mlcp_FB()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

### Direct + Nonsmooth Newton solver, Fisher-Burmeister (SICONOS_MLCP_DIRECT_FB)¶

Try direct solver then switch to Fisher-Burmeister.

driver: mlcp_direct_FB()

parameters:

• iparam[SICONOS_IPARAM_MAX_ITER] = 10000

• dparam[SICONOS_DPARAM_TOL] = 1e-12

• iparam[SICONOS_IPARAM_MLCP_NUMBER_OF_CONFIGURATIONS]

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_NEG];

• dparam[SICONOS_DPARAM_MLCP_SIGN_TOL_POS];

• iparam[SICONOS_IPARAM_MLCP_UPDATE_REQUIRED]

return iparam for iters