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December  2019, 24(12): 6745-6770. doi: 10.3934/dcdsb.2019165

## Semidefinite approximations of invariant measures for polynomial systems

 1 CNRS; LAAS; Université de Toulouse, 7 avenue du colonel Roche, F-31400 Toulouse, France 2 Universidad de la República, CURE, Maldonado, Uruguay 3 Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 4, CZ-16206 Prague, Czechia

* Corresponding author: Victor Magron

Received  August 2018 Revised  March 2019 Published  July 2019

Fund Project: The first author benefited from the support of the FMJH Program PGMO (EPICS project) and EDF, Thales, Orange et Criteo, as well as from the Tremplin ERC Stg Grant ANR-18-ERC2-0004-01 (T-COPS project). The research of the third author was funded by the European Research Council (ERC) under the European's Union Horizon 2020 research and innovation program (grant agreement 666981 TAMING project).

We consider the problem of approximating numerically the moments and the supports of measures which are invariant with respect to the dynamics of continuous- and discrete-time polynomial systems, under semialgebraic set constraints. First, we address the problem of approximating the density and hence the support of an invariant measure which is absolutely continuous with respect to the Lebesgue measure. Then, we focus on the approximation of the support of an invariant measure which is singular with respect to the Lebesgue measure.

Each problem is handled through an appropriate reformulation into a conic optimization problem over measures, solved in practice with two hierarchies of finite-dimensional semidefinite moment-sum-of-square relaxations, also called Lasserre hierarchies.

Under specific assumptions, the first Lasserre hierarchy allows to approximate the moments of an absolutely continuous invariant measure as close as desired and to extract a sequence of polynomials converging weakly to the density of this measure.

The second Lasserre hierarchy allows to approximate as close as desired in the Hausdorff metric the support of a singular invariant measure with the level sets of the Christoffel polynomials associated to the moment matrices of this measure.

We also present some application examples together with numerical results for several dynamical systems admitting either absolutely continuous or singular invariant measures.

Citation: Victor Magron, Marcelo Forets, Didier Henrion. Semidefinite approximations of invariant measures for polynomial systems. Discrete & Continuous Dynamical Systems - B, 2019, 24 (12) : 6745-6770. doi: 10.3934/dcdsb.2019165
##### References:

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##### References:
Approximate invariant density for the dynamics from (29) with corresponding approximations $h_2^r$ (solid curve) of the exact density $h^\star$ (dashed curve) for $r \in \{4,6,8\}$ and $w = \frac{\sqrt{99}}{10}$
Approximate invariant density for the piecewise systems defined respectively in (31), (32) and (33) with corresponding approximations $h_\infty^r$ (solid curve) of the exact density $h^\star$ (dashed curve)
Hénon attractor (blue) and approximations ${\bf{S}}^r$ (light gray) for the support of the invariant measure w.r.t. the map from Example 5.2.1 for $r \in \{4,6,8\}$, $a = 1.4$ and $b = 0.3$
Van der Pol attractor (blue) and approximations ${\bf{S}}^r$ (light gray) for the support of the invariant measure w.r.t. the map from Example 5.2.2 for $r \in \{4,6,8\}$ and $a = 0.5$
Arneodo-Coullet attractor (blue) and approximations ${\bf{S}}^4$ (red) for the support of the invariant measure w.r.t. the map from Example 5.2.3 for $a = -5.5$, $b = 3.5$ and $c = -1$
Comparison of timing results for the dynamics from (29)
 relaxation order $r$ 4 6 8 moments 70 168 330 variables 245 1035 3199 time (s) 0.89 1.05 1.37
 relaxation order $r$ 4 6 8 moments 70 168 330 variables 245 1035 3199 time (s) 0.89 1.05 1.37
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