Convex relaxation and variational approximation of functionals defined on 1-d connected sets M. Bonafini 1 G. Orlandi 1 E. Oudet 3 1 (Verona), 3 (Grenoble) BIRS Banff, May 2, 2017 M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
The Steiner Tree Problem Steiner Tree Problem: Given N points P i ∈ X in a metric space, (e.g. X a graph, with P i given vertices), find a connected (sub-)graph F ⊂ X containing the points P i and having minimal length. An optimal graph F is called a Steiner Minimal Tree (SMT). Examples. X = R k : Euclidean (or geometric) STP (design of optimal transport channels /networks w.r.t. given terminal points) X ⊂ G ⊂ R k (contained) in a fixed grid G (or X ⊂ R k endowed with the ℓ 1 metric): rectilinear STP (optimal design of net routing in VLSI circuits for k = 2 , 3). Euclidean STP is a NP-hard problem. Existence of PTAS, especially developed in case k = 2. M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Euclidean STP - features of solutions Acyclic graph, max N − 2 Steiner points (incident angles ≡ 120 ◦ ) No additional Steiner points ⇔ SMT ≡ MST (Minimal Spanning Tree, easy to compute) √ Steiner ratio (MST/SMT) in R 2 : 2 / 3 (euclidean, open conj.), 2 / 3 (rectilinear) M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Set formulation in metric spaces Formulation of the STP in a metric space X [Paolini-Stepanov]: given A ⊂ X a compact (possibly infinite) set of terminal points, ( STP ) ≡ inf {H 1 ( S ) , S connected , S ⊃ A } Existence relies on Golab compactness theorem for compact connected sets. Allows for even further generalizations (e.g. inf H 1 ( S ) , S ∪ A connected). Functional framework not easy for computations. M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Formulation for measures STP vs Branched Optimal Transport. Formulation for measures instead of sets: the network S connecting the P i is made by streamlines of a vector measure (current) µ = θ ( x ) τ S ( x ) · H 1 S flowing unit masses located at P i , i < N , to P N . The transport cost is a sublinear (concave) function of the mass density, to favour branching [Xia]. For 0 < α ≤ 1, � � � N − 1 � | θ | α ( x ) d H 1 ( x ) , div µ = ( N − 1 ) δ P N − ( M α ) ≡ inf M α ( µ ) = δ P i S i = 1 Rmk. ( M 1 ) is well-behaved, as a mass minimization problem, i.e. the minimization of the total variation norm M 1 ( µ ) = || µ || , it corresponds to an Optimal Transport Pb. with L 1 cost, (cf. Beckmann Pb.) while ( M 0 ) ≡ ( STP ) corresponds to size minimization (minimizing sequences a priori non compact). Existence for α > 0: [Xia], [Bernot-Caselles-Morel], [Depauw-Hardt]. M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Formulation for measures Expected convergence ( M α ) → ( M 0 ) as α → 0 (cf. [Marchese-Massacesi]) (picture from [Oudet-Santambrogio]) M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Approximations for ( M α ) and ( M 0 ) in R 2 Approximation of ( M 0 ) in R 2 by F ǫ ( µ ) = M 0 ( µ ) + ǫ 2 M 1 ( µ ) [Depauw-Hardt], [Morgan] Variational approximation (via Γ -convergence) of ( M α ) through phase transition functionals defined for u ∈ H 1 ( R 2 ; R 2 ) [Oudet-Santambrogio] � � R 2 | u | β + ǫ α + 1 M α,ǫ ( u ) = ǫ α − 1 R 2 |∇ u | 2 ( div u = ρ 0 − ρ 1 ) Approximation of minimizers of ( M 0 ) by minimizers of phase transition functionals in R 2 [Bonnivard-Lemenant-Santambrogio], [Millot & al.], [Chambolle-Merlet-Ferrari] � � � N F ǫ ( ρ ) = 1 R 2 |∇ ρ | 2 + 1 R 2 ( 1 − ρ ) 2 + ǫ d ρ ( x i , x N ) 4 ǫ c ǫ i = 1 � γ ρ ( x ) d H 1 ( x ) , γ ( 0 ) = x i , γ ( 1 ) = x N } . Level where d ρ ( x i , x N ) = inf { sets { d ρ = 0 } are connected and d ρ ǫ → d with { d = 0 } ≡ SMT. M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Optimal partitions in R 2 If P i ∈ ∂ Ω , Ω ⊂ R 2 convex, (STP) is related to a minimal partition problem, e.g. � inf { Ω |∇ u | , u ∈ BV (Ω; { e 1 , ..., e N } ) , u | ∂ Ω = u 0 } [Ambrosio-Braides] Variants, approximations, convex relaxation and dual formulation: [Otto et al.], [Oudet], [Bretin et al.], [Chambolle-Cremers-Pock] M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Plateau Problem in covering spaces Interpretation of area minimizing surfaces as solutions of a Plateau problem for currents in a suitable covering space of R k . Use of the calibration method [Brakke], The case k = 2 corresponds to STP: analysis and variational approximation [Bellettini-Amato-Paolini] Caibrations [Pluda-Carioni] M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Variational formulations of STP Plateau Problem for polyhedral chains Let’s try to formulate (STP) as a Plateau problem for polyhedral (or rectifiable) 1-chains T = � L i with ∂ T = � a i P i Integer multiplicities a i ∈ Z are not suited: Plateau problem corresponds to an OT problem ( M 1 ) ≡ inf || T || , with || T || = � | L i | . Some examples of troubles: non connectedness, no Steiner (branching) points... M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Plateau problem for G -currents vs STP The approach of [Marchese-Massaccesi] Let’s try with a more general discrete coefficient group G : what should be the requirements on G ? G normed abelian group (e.g. G < E additive subgroup of a Banach space E ) � � � N � T = γ j L j , ∂ T = γ j ∂ L j = g i P i , g i ∈ G , γ j = g i i = 1 i ∈ Λ j � � || T || = || γ j || · | L j | , || γ j || = 1 ∀ j ⇒ || T || = | L i | � N � N g i P i = ∂ T ⇐ ⇒ g i = 0 ( boundary ) i = 1 i = 1 � g i � = 0 ∀ Λ ⊂ { 1 , ..., N } , Λ � = { 1 , ..., N } ( connectedness ) i ∈ Λ || � i ∈ Λ g i || = 1 ensures both connectedness and || T || = � | L i | . M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Plateau problem for G -currents vs STP The approach of [Marchese-Massaccesi] Consider for example E = R N − 1 , G = Z N − 1 , g i = e i for i = 1 , ..., N − 1, g N = − � N − 1 i = 1 e i Remark. Endowing E + (positive orthant of E ) with the ℓ ∞ norm fulfills all previous requirements! M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Plateau problem for G -currents vs ( M α ) The approach of [Marchese-Massaccesi] Remark. Endowing E + with the ℓ q norm fulfills all requirements for an equivalent formulation of the irrigation problem ( M α ) , with α = q − 1 . Remark. any norm on E that coincides with ℓ ∞ (resp. ℓ q ) on E + is suited to handle ( M 0 ) ≡ STP (resp. ( M α ) . A natural choice, having in mind optimal convex relaxations of the problem, is to considet the largest possible extension to E (convex | E + (resp. ℓ q 1-homogeneous envelope) of ℓ ∞ | E + ) This envelope coincides with the norm introduced by [Marchese-Massaccesi] to study ( M 0 ) (resp. ( M α ) ) via the calibration method. M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Rectifiable G-currents Original definition by [Fleming], generalization to metric spaces by [Depauw-Hardt]. We follow [Marchese-Massaccesi]. Let G < E be a discrete subgroup of a ( m -dimensional) Banach space E , R ⊂ R k a (closed) d -rectifiable set, τ ( x ) ∈ Λ ∗ ( R k ) a H d -measurable orientation for R (a unit simple d -vectorfield tangent to R ), and g ( x ) : R → G ⊂ E a H d -measurable G -valued multiplicity function defined on R . The vector measure T (with spt T = R ) T ≡ T ( g , τ, R ) ≡ g ( x ) ⊗ τ ( x ) · H d R c ) ∗ of polyhedral G -chains. is a rectifiable G -current. It is a limit in ( C 1 Rmk. If e j , j = 1 , ..., m is a basis for E , with || e j || = 1, then we may write g ( x ) = � j g j ( x ) e j with g j ( x ) ∈ Z and accordingly T = � j T j e j , with T j = g j ( x ) τ ( x ) · H d R a (cassical) rectifiable current. M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
Normal (resp. integral) E- (resp. G-) currents E -currents T are defined by duality with smooth compactly supported E ∗ -valued forms ω ( x ) = θ ( x ) ⊗ φ ( x ) = � j ω j ( x ) e j ∈ E ∗ ⊗ Λ ∗ ( R k ) . Exterior derivative d ω ( x ) = � j d ω j ( x ) e j Mass norm || T || = sup { T ( ω ) , || θ || E ∗ ≤ 1 , || φ || ∗ ≤ 1 } Rmk. ∂ T = � j ∂ T j e j . Boundary ∂ T ( ω ) = T ( d ω ) . || T || < + ∞ ⇒ T = ( � i g i ⊗ τ i ) | µ T | , � � T ( ω ) = i � θ ( x ) , g i ( x ) � · � φ ( x ) , τ i ( x ) � d | µ T | Normal currents: N ( T ) = || T || + || ∂ T || < + ∞ Integral currents: both T and ∂ T rectifiable G -currents � sptT � θ ( x ) , g ( x ) � · � φ ( x ) , τ ( x ) � d H d ( x ) T ( ω ) = � sptT || g ( x ) || d H d ( x ) || T || = c ) ∗ closure and compactness theorem for N -bdd normal and ( C 1 integral currents: apply componentwise [Federer-Fleming] M. Bonafini, G. Orlandi , E. Oudet Euclidean Steiner tree problem 2/91/17
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