Initial value problems by convex minimization and matrix-valued - PowerPoint PPT Presentation

Initial value problems by convex minimization and matrix-valued optimal transport Yann Brenier CNRS, DMA/ENS, 45 rue d’Ulm, FR-75005 Paris, in association with the CNRS-INRIA team "MOKAPLAN". 2020 Fields Medal Symposium ALESSIO FIGALLI YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 1 / 21

SOLVING IVP BY CONVEX MINIMIZATION Solving initial value problems by convex minimization is an old idea going back to the least square method for linear equations. For nonlinear systems there has been many contributions, including Brezis-Ekeland, Ghoussoub, Mielke-Stefanelli, Visintin, etc... Recently, we introduced another approach, working for systems of conservation laws with a convex entropy. cf. Y.B. CMP 2018, followed by D. Vorotnikov arXiv:1905.060592. TOPICS OF TODAY: let us try to apply this method to some parabolic equations: porous medium, viscous Hamilton-Jacobi and incompressible Navier-Stokes. YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 2 / 21

I. The quadratic porous medium equation (QPME) ∂ t u = ∆ u 2 / 2 , x ∈ T d , u = u ( t , x ) ≥ 0 , t ≥ 0 , which is nothing but the macroscopic limit of the properly rescaled (deterministic) system of particles: ( X k − X j ) exp ( −| X k − X j | 2 dX k = ǫ − 1 � ) , dt ǫ j = 1 , N u ( t , x ) ∼ 1 1 / N << ǫ d << 1 . � δ ( x − X j ( t )) , N j = 1 , N cf. P .-L. Lions, S. Mas-Gallic 2001 and ...A. Figalli, R. Philipowski 2008 . YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 3 / 21

A strange minimization problem. We start with the rather absurd problem of minimizing the time integral of the "entropy" � u 2 ( t , x ) dxdt , Q = [ 0 , T ] × T d , Q among weak solutions ot the QPME ∂ t u = ∆ u 2 / 2 , x ∈ T d . u = u ( t , x ) ∈ R , t ≥ 0 , with a given initial condition u 0 ≥ 0 in L ∞ ( T d ) . YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 4 / 21

The saddle point formulation reads � u 2 − 2 ∂ t φ u − ∆ φ u 2 + 2 u 0 ∂ t φ � � I ( u 0 ) = inf u sup , φ Q where the only constraints are: i) for test function φ to be smooth and vanish at t = T ; ii) for function u to be square integrable on Q . This problem admits an interesting concave relaxation: � u 2 − 2 ∂ t φ u − ∆ φ u 2 + 2 u 0 ∂ t φ � � J ( u 0 ) = sup inf . u φ Q YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 5 / 21

The relaxed problem is very simple � u 2 − 2 ∂ t φ u − ∆ φ u 2 + 2 u 0 ∂ t φ � � J ( u 0 ) = sup inf = u φ Q − ( ∂ t φ ) 2 � � � sup 1 − ∆ φ + 2 u 0 ∂ t φ , ∆ φ ≤ 1 , φ ( T , · ) = 0 . φ Q Setting q = ∂ t φ , σ = 1 − ∆ φ , we get: J ( u 0 ) = − q 2 � � � sup σ + 2 u 0 q , ∂ t σ + ∆ q = 0 , σ ( T , · ) = 1 σ, q Q YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 6 / 21

Interestingly enough, this optimisation problem − q 2 � � � sup σ + 2 u 0 q , σ, q Q s.t. ∂ t σ + ∆ q = 0 , σ ( T , · ) = 1 , is (at least as d = 1) almost the same as the recent formulation "à la Benamou-Brenier" proposed by Huesmann and Trevisan for the time-discrete martingale optimal transport problem. (See also Ghoussoub-Kim.) YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 7 / 21

All solutions of the QPME satisfy the Aronson-Bénilan estimate ∆ u ≥ − κ/ t where κ just depends on d . Let us try to find a solution φ to the concave optimization problem just by solving the final VP ∂ t φ = ( 1 − ∆ φ ) u , φ ( T , · ) = 0 , i.e., for α = 1 − ∆ φ : ∂ t α + ∆( α u ) = 0 , α ( T , · ) = 1 . From Aronson-Bénilan, we deduce α ( t , x ) ≥ ( t / T ) κ . YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 8 / 21

Proof. (Assuming u to be smooth) we have ∂ t α + ∆( α u ) = ∂ t α + u ∆ α + 2 ∇ α · ∇ u + α ∆ u = 0 . Thanks to AB, we get for A ( t ) = inf x ∈ T d α ( t , x ) A ′ ( t ) ≤ κ A ( t ) / t . So, log A ( T ) − log A ( t ) ≤ κ ( log T − log t ) , and therefore A ( t ) ≥ ( t / T ) κ (since A ( T ) = 1). End of proof. YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 9 / 21

Optimality of φ . Let us now evaluate − ( ∂ t φ ) 2 � � � j = 1 − ∆ φ + 2 u 0 ∂ t φ . Q Since u solves the QPME with initial condition u 0 , 2 ∂ t φ u + ∆ φ u 2 − 2 ∂ t φ u 0 � � � we have = 0 . Thus Q − ( ∂ t φ ) 2 � � � � 1 − ∆ φ + 2 u ∂ t φ + ∆ φ u 2 u 2 j = = Q Q (using ∂ t φ = ( 1 − ∆ φ ) u ) which shows that φ is Q u 2 ≥ I ( u 0 ) ≥ J ( u 0 ) . � optimal since J ( u 0 ) ≥ j = YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 10 / 21

II. The viscous Hamilton-Jacobi equation ∂ t φ + 1 2 |∇ φ | 2 = ǫ ∆ φ = 0 , D = T d , on Q = [ 0 , T ] × D , φ ( 0 , · ) = φ 0 . Q | B | 2 among all weak solutions of � Minimize ∂ t B + ∇ ( | B | 2 − ǫ ∇ · B ) = 0 , B ( 0 , · ) = B 0 = ∇ φ 0 . 2 The concave dual problem turns out to be: q · B 0 + | q − ǫ ∇ ρ | 2 � inf 2 ρ ρ, q Q where the fields ρ ≥ 0, q ∈ R d are constrained by ∂ t ρ + ∇ · q = 0 , ρ ( T , · ) = 1 . YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 11 / 21

The resulting problem can also be written | q | 2 + ǫ 2 |∇ ρ | 2 � inf 2 ρ ρ, q Q � + ρ ( 0 , · )( ǫ log ρ ( 0 , · ) + φ 0 ) , s . t . ∂ t ρ + ∇ · q = 0 , ρ ( T , · ) = 1 , D i.e. as a variant of the "Schrödinger problem", a noisy version of the optimal transport problem with quadratic cost, intensively studied in the recent years, after Ch. Léonard, e.g. in the CNRS-INRIA MOKAPLAN team (mostly for numerical purposes), and very recently by A. Baradat, and L. Monsaingeon. YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 12 / 21

III. Navier-Stokes equation Q | v | 2 among all weak solutions of � Now, we minimize ∂ t v + ∇ · ( v ⊗ v ) + ∇ p = ǫ ∆ v , ∇ · v = 0 , v ( 0 , · ) = v 0 , and get by duality the convex minimization problem: � ( q − ǫ ∇ · M ) · M − 1 · ( q − ǫ ∇ · M ) − 2 q · v 0 inf M , q Q where Q = [ 0 , T ] × T d , the matrix-valued field M = M T ≥ 0 and the vector field q being subject to ∂ t M + ∇ q + ∇ q T = 2 D 2 ∆ − 1 ∇ · q , M ( T , · ) = I d . YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 13 / 21

Final remarks 1) The Schrödinger problem (1931) is closely related to the Schrödinger equation (1925), which can be solved by looking at critical points ( ρ, q ) of the following action (featuring a crucial change of sign): | q | 2 −|∇ ρ | 2 � s . t . ∂ t ρ + ∇ · q = 0 , 2 ρ Q through the Madelung transform (1926): ψ = √ ρ e i θ , q = ρ ∇ θ. YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 14 / 21

Final remarks (continued) 2) The optimization problem we obtained from the NS equations can be seen as a (very special) example of a matrix-valued Optimal Transport problem (*), for which we may refer to a collection of works by Tryphon Georgiou and coll., and a recent paper by Y.B. and Dmitry Vorotnikov (SIMA 2020). (*) due to the special structure of its time-boundary conditions, the NS optimization problem more precisely corresponds to a matrix-valued Mean-Field Game problem. YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 15 / 21

Final remarks (continued) 3) In the NS optimization problem features a matrix-valued "Fisher information" ( ∇ · M ) · M − 1 · ( ∇ · M ) , M = M T ≥ 0 , very roughly similar to the 4D-Einstein action (*) ij g ij Γ k ik g ij Γ k (Γ m km − Γ m � jm ) − det g where g ij is Lorentzian of inverse g ij and connection Γ i jk = g im ( ∂ j g km + ∂ k g jm − ∂ m g kj ) / 2 . * Note that General Relativity has been recently related to Optimal Transportation (in particular by R. McCann arXiv:1808.01536, A. Mondino, S. Suhr arXiv:1810.13309). YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 16 / 21

Final remarks 4) The convex optimization problem we have just derived from the NS equations is closely related to the "Brödinger (or Bredinger) problem" investigated by Arnaudon, Cruzeiro, Léonard, Zambrini, and more recently by Baradat and Monsaingeon. This problem can be interpreted as the stochastic version of the "incompressible optimal transport problem" studied by Y.B. 1990/1993/1999 and L. Ambrosio-A. Figalli 2008, in connection with the incompressible Euler equations. YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 17 / 21

AND NOW A QUESTION TO ALESSIO AND FRANCESCO. WHAT ABOUT A QUANTITATIVE VERSION OF THE FOLLOWING: YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 18 / 21

YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 19 / 21

YB (CNRS/DMA-ENS, Paris.) IVP by convex optimization Fields Institute 23 Oct 2020 20 / 21