Uniqueness of the FisherRao metric on the space of smooth densities - PowerPoint PPT Presentation

Uniqueness of the Fisher–Rao metric on the space of smooth densities Peter W. Michor University of Vienna, Austria www.mat.univie.ac.at/˜michor IGAIA IV Information Geometry and its Applications IV June 12-17, 2016, Liblice, Czech Republic In honor of Shun-ichi Amari

Based on: [M.Bauer, M.Bruveris, P.Michor: Uniqueness of the Fisher–Rao metric on the space of smooth densities, Bull. London Math. Soc. doi:10.1112/blms/bdw020] [M.Bruveris, P.Michor: Geometry of the Fisher-Rao metric on the space of smooth densities] [M.Bruveris, P. Michor, A.Parusinski, A. Rainer: Moser’s Theorem for manifolds with corners, arxiv:1604.07787] [M.Bruveris,P.Michor, A.Rainer: Determination of all diffeomorphism invariant tensor fields on the space of smooth positive densities on a compact manifold with corners] The infinite dimensional geometry used here is based on: [Andreas Kriegl, Peter W. Michor: The Convenient Setting of Global Analysis. Mathematical Surveys and Monographs, Volume: 53, Amer. Math. Soc., 1997] Wikipedia [https://en.wikipedia.org/wiki/Convenient vector space]

Abstract For a smooth compact manifold M , any weak Riemannian metric on the space of smooth positive densities which is invariant under the right action of the diffeomorphism group Diff ( M ) is of the form � � � α β G µ ( α, β ) = C 1 ( µ ( M )) µµ + C 2 ( µ ( M )) α · β µ M M M � for smooth functions C 1 , C 2 of the total volume µ ( M ) = M µ . In this talk the result is extended to: (0) Geometry of the Fisher-Rao metric: geodesics and curvature. (1) manifolds with boundary, for manifolds with corner. (2) to tensor fields of the form G µ ( α 1 , α 2 , . . . , α k ) for any k which are invariant under Diff ( M ).

The Fisher–Rao metric on the space Prob( M ) of probability densities is of importance in the field of information geometry. Restricted to finite-dimensional submanifolds of Prob( M ), so-called statistical manifolds, it is called Fisher’s information metric [Amari: Differential-geometrical methods in statistics, 1985]. The Fisher–Rao metric is invariant under the action of the diffeomorphism group. A uniqueness result was established [ˇ Cencov: Statistical decision rules and optimal inference, 1982, p. 156] for Fisher’s information metric on finite sample spaces and [Ay, Jost, Le, Schwachh¨ ofer, 2014] extended it to infinite sample spaces. The Fisher–Rao metric on the infinite-dimensional manifold of all positive probability densities was studied in [Friedrich: Die Fisher-Information und symplektische Strukturen, 1991], including the computation of its curvature.

� � The space of densities Let M m be a smooth manifold. Let ( U α , u α ) be a smooth atlas for it. The volume bundle (Vol( M ) , π M , M ) of M is the 1-dimensional vector bundle (line bundle) which is given by the following cocycle of transition functions: ψ αβ : U αβ = U α ∩ U β → R \ { 0 } = GL (1 , R ) , 1 ψ αβ ( x ) = | det d ( u β ◦ u − 1 α )( u α ( x )) | = β )( u β ( x )) | . | det d ( u α ◦ u − 1 Vol(M) is a trivial line bundle over M . But there is no natural trivialization. There is a natural order on each fiber. Since Vol( M ) is a natural bundle of order 1 on M , there is a natural action of the group Diff( M ) on Vol( M ), given by | det( T ϕ − 1 ) | ◦ ϕ � Vol(M) Vol(M) . ϕ � M M

If M is orientable, then Vol( M ) = Λ m T ∗ M . If M is not orientable, let ˜ M be the orientable double cover of M with its deck-transformation τ : ˜ M → ˜ M . Then Γ(Vol( M )) is isomorphic to the space { ω ∈ Ω m ( ˜ M ) : τ ∗ ω = − ω } . These are the ‘formes impaires’ of de Rham. See [M 2008, 13.1] for this. Sections of the line bundle Vol( M ) are called densities. The space Γ(Vol( M )) of all smooth sections is a Fr´ echet space in its natural topology; see [Kriegl-M, 1997]. For each section α of Vol( M ) of � compact support the integral M α is invariantly defined as follows: Let ( U α , u α ) be an atlas on M with associated trivialization ψ α : Vol( M ) | U α → R , and let f α be a partition of unity with supp( f α ) ⊂ U α . Then we put � � � � � f α ( u − 1 α ( y )) .ψ α ( µ ( u − 1 µ = f α µ := α ( y ))) dy . M U α u α ( U α ) α α The integral is independent of the choice of the atlas and the partition of unity.

The Fisher–Rao metric Let M m be a smooth compact manifold without boundary. Let Dens + ( M ) be the space of smooth positive densities on M , i.e., Dens + ( M ) = { µ ∈ Γ(Vol( M )) : µ ( x ) > 0 ∀ x ∈ M } . Let Prob( M ) be the subspace of positive densities with integral 1. For µ ∈ Dens + ( M ) we have T µ Dens + ( M ) = Γ(Vol( M )) and for µ ∈ Prob( M ) we have � T µ Prob( M ) = { α ∈ Γ(Vol( M )) : M α = 0 } . The Fisher–Rao metric on Prob( M ) is defined as: � α β G FR µ ( α, β ) = µµ. µ M It is invariant for the action of Diff( M ) on Prob( M ): � ( ϕ ∗ ) ∗ G FR � µ ( α, β ) = G FR ϕ ∗ µ ( ϕ ∗ α, ϕ ∗ β ) = � � � α �� β � α β ϕ ∗ µ = = µ ◦ ϕ µ ◦ ϕ µµ . µ M M

Theorem [BBM, 2016] Let M be a compact manifold without boundary of dimension ≥ 2 . Let G be a smooth (equivalently, bounded) bilinear form on Dens + ( M ) which is invariant under the action of Diff( M ) . Then � � � α β G µ ( α, β ) = C 1 ( µ ( M )) µ µ + C 2 ( µ ( M )) α · β µ M M M for smooth functions C 1 , C 2 of the total volume µ ( M ) . To see that this theorem implies the uniqueness of the Fisher–Rao metric, note that if G is a Diff( M )-invariant Riemannian metric on Prob( M ), then we can equivariantly extend it to Dens + ( M ) via � � � � � � � � µ µ G µ ( α, β ) = G α − α µ ( M ) , β − β . µ µ ( M ) µ ( M ) M M

Relations to right-invariant metrics on diffeom. groups Let µ 0 ∈ Prob( M ) be a fixed smooth probability density. In [Khesin, Lenells, Misiolek, Preston, 2013] it has been shown, that � the degenerate, ˙ H 1 -metric 1 M div µ 0 ( X ) . div µ 0 ( X ) .µ 0 on X ( M ) is 2 invariant under the adjoint action of Diff( M , µ 0 ). Thus the induced degenerate right invariant metric on Diff( M ) descends to a metric on Prob( M ) ∼ = Diff( M , µ 0 ) \ Diff( M ) via Diff( M ) ∋ ϕ �→ ϕ ∗ µ 0 ∈ Prob( M ) which is invariant under the right action of Diff( M ). This is the Fisher–Rao metric on Prob( M ). In [Modin, 2014], the ˙ H 1 -metric was extended to a non-degenerate metric on Diff( M ), also descending to the Fisher–Rao metric.

Corollary. Let dim( M ) ≥ 2 . If a weak right-invariant (possibly degenerate) Riemannian metric ˜ G on Diff( M ) descends to a metric G on Prob( M ) via the right action, i.e., the mapping ϕ �→ ϕ ∗ µ 0 from (Diff( M ) , ˜ G ) to (Prob( M ) , G ) is a Riemannian submersion, then G has to be a multiple of the Fisher–Rao metric. Note that any right invariant metric ˜ G on Diff( M ) descends to a metric on Prob( M ) via ϕ �→ ϕ ∗ µ 0 ; but this is not Diff( M )-invariant in general.

Invariant metrics on Dens + ( S 1 ). Dens + ( S 1 ) = Ω 1 + ( S 1 ), and Dens + ( S 1 ) is Diff( S 1 )-equivariantly isomorphic to the space of all Riemannian metrics on S 1 via ) 2 : Dens + ( S 1 ) → Met( S 1 ), Φ( fd θ ) = f 2 d θ 2 . Φ = ( On Met( S 1 ) there are many Diff( S 1 )-invariant metrics; see [Bauer, Harms, M, 2013]. For example Sobolev-type metrics. Write gd θ 2 and h = ˜ kd θ 2 with hd θ 2 , k = ˜ g ∈ Met( S 1 ) in the form g = ˜ g , ˜ h , ˜ k ∈ C ∞ ( S 1 ). The following metrics are Diff( S 1 )-invariant: ˜ � ˜ � � ˜ � h k G l g . (1 + ∆ g ) n g ( h , k ) = g d θ ; ˜ ˜ g ˜ S 1 here ∆ g is the Laplacian on S 1 with respect to the metric g . The pullback by Φ yields a Diff( S 1 )-invariant metric on Dens + ( M ): � 1 + ∆ Φ( µ ) � n � β � � α G µ ( α, β ) = 4 µ. µ . µ S 1 For n = 0 this is 4 times the Fisher–Rao metric. For n ≥ 1 we get different Diff( S 1 )-invariant metrics on Dens + ( M ) and on Prob( S 1 ).

Main Theorem Let M be a compact manifold, possibly with corners, of dimension � 0 � ≥ 2 . Let G be a smooth (equivalently, bounded) -tensor field n on Dens + ( M ) which is invariant under the action of Diff( M ) . If M is not orientable or if n ≤ dim( M ) = m, then � α 1 µ . . . α n G µ ( α 1 , . . . , α n ) = C 0 ( µ ( M )) µ µ M � � n � α 1 µ . . . � α i µ . . . α n + C i ( µ ( M )) α i · µ µ M M i =1 � � n � α i α j α 1 µ . . . � α i µ . . . � α i µ . . . α n + C ij ( µ ( M )) µ µ · µ µ µ M M i < j + . . . � � � α 1 α 2 α n + C 12 ... n ( µ ( M )) µ µ · µ µ · · · · · µ µ · M M M for some smooth functions C 0 , . . . of the total volume µ ( M ) .

Main Theorem, continued If M is orientable and n > dim( M ) = m, then each integral over more than m functions α i /µ has to be replaced by the following expression which we write only for the first term: � α 1 µ . . . α n C 0 ( µ ( M )) µ µ + M � α k 1 � � α k n − m +1 � � α k n � µ . . . α k n − m C K + 0 ( µ ( M )) d ∧ · · · ∧ d µ µ µ where K = { k n − m +1 , . . . , k n } runs through all subsets of { 1 , . . . , n } containing exactly m elements.