# Schedule for: 18w5151 - Shape Analysis, Stochastic Mechanics and Optimal Transport

Beginning on Sunday, December 9 and ending Friday December 14, 2018

All times in Banff, Alberta time, MST (UTC-7).

Sunday, December 9 | |
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16:00 - 17:30 | Check-in begins at 16:00 on Sunday and is open 24 hours (Front Desk - Professional Development Centre) |

17:30 - 19:30 |
Dinner ↓ A buffet dinner is served daily between 5:30pm and 7:30pm in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |

20:00 - 22:00 | Informal gathering (Corbett Hall Lounge (CH 2110)) |

Monday, December 10 | |
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07:00 - 08:45 |
Breakfast ↓ Breakfast is served daily between 7 and 9am in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |

08:45 - 09:00 |
Introduction and Welcome by BIRS Staff ↓ A brief introduction to BIRS with important logistical information, technology instruction, and opportunity for participants to ask questions. (TCPL 201) |

09:00 - 09:40 |
Boris Khesin: Beyond Arnold’s geodesic framework of an ideal hydrodynamics ↓ We discuss a ramification of Arnold’s group-theoretic approach to ideal hydrodynamics as the geodesic flow for a right-invariant metric on the group of volume-preserving diffeomorphisms. We show such problems of mathematical physics as the motion of vortex sheets or fluids with moving boundary, have Lie groupoid, rather than Lie group, symmetries, and describe the corresponding geometry and equations. (This is a joint work with Anton Izosimov.) (TCPL 201) |

09:40 - 10:20 |
Gerard Misiolek: The $L^2$ exponential map in 2D and 3D hydrodynamics ↓ In the 1960's V. Arnold showed how solutions of the incompressible Euler equations can be viewed as geodesics on the group of diffeomorphisms of the fluid domain equipped with a metric given by fluid's kinetic energy. The study of the exponential map of this metric is of particular interest and I will describe recent results concerning its properties as well as some necessary background. (TCPL 201) |

10:20 - 10:50 | Coffee Break (TCPL Foyer) |

10:50 - 11:30 |
Klas Modin: Semi-invariant metrics on diffeos ↓ We investigate a generalization of cubic splines to Riemannian manifolds.
Spline curves are defined as minimizers of the spline energy---a combination of the Riemannian path energy and
the time integral of the squared covariant derivative of the path velocity---under suitable interpolation conditions.
A variational time discretization for the spline energy leads to a constrained optimization problem over discrete paths on the manifold.
Existence of continuous and discrete spline curves is established using the direct method in the calculus of variations.
Furthermore, the convergence of discrete spline paths to a continuous spline curve
follows from the $\Gamma$-convergence of the discrete to the continuous spline energy.
Finally, selected example settings are discussed, including splines on embedded finite-dimensional manifolds,
on a high-dimensional manifold of discrete shells with applications in surface processing,
and on the infinite-dimensional shape manifold of viscous rods. (TCPL 201) |

11:30 - 13:00 |
Lunch ↓ Lunch is served daily between 11:30am and 1:30pm in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |

13:00 - 14:00 |
Guided Tour of The Banff Centre ↓ Meet in the Corbett Hall Lounge for a guided tour of The Banff Centre campus. (Corbett Hall Lounge (CH 2110)) |

14:00 - 14:20 |
Group Photo ↓ Meet in foyer of TCPL to participate in the BIRS group photo. The photograph will be taken outdoors, so dress appropriately for the weather. Please don't be late, or you might not be in the official group photo! (TCPL 201) |

14:20 - 15:00 |
Ana Cruzeiro: On some relations between Optimal Transport and Stochastic Geometric Mechanics ↓ We formulate the so-called Schrodinger problem in Optimal Transport on lie group and derive the corresponding Euler-Poincaré equations. (TCPL 201) |

15:00 - 15:20 | Coffee Break (TCPL Foyer) |

15:20 - 16:00 |
Christian Léonard: Some ideas and results about gradient flows and large deviations ↓ In several situations, the empirical measure of a large number of random particles evolving in a heat bath is an approximation of the solution of a dissipative PDE. The evaluation of the probabilities of large deviations of this empirical measure suggests a way of defining a natural ``large deviation cost'' for these fluctuations, very much in the spirit of optimal transport. Some standard Wasserstein gradient flow evolutions are revisited in this perspective, both in terms of heuristic results and a few rigorous ones.
This talk gathers several joint works with Julio Backhoff, Giovanni Conforti, Ivan Gentil, Luigia Ripani and Johannes Zimmer. (TCPL 201) |

16:00 - 16:40 |
Marc Arnaudon: A duality formula and a particle Gibbs sampler for continuous time Feynman-Kac measures on path spaces ↓ "Continuous time Feynman-Kac measures on path spaces are central in
applied probability, partial differential equation theory, as well as in quantum
physics. I will present a new duality formula between normalized Feynman-Kac
distribution and their mean field particle interpretations. Among others, this formula
will allow to design a reversible particle Gibbs-Glauber sampler for continuous time
Feynman-Kac integration on path spaces. This result extends the particle Gibbs
samplers introduced by Andrieu-Doucet-Holenstein in the context of discrete
generation models to continuous time Feynman-Kac models and their interacting
jump particle interpretations. I will also provide new propagation of chaos estimates
for continuous time genealogical tree based particle models with respect to the time
horizon and the size of the systems. These results allow to obtain sharp quantitative
estimates of the convergence rate to equilibrium of particle Gibbs-Glauber samplers. " (TCPL 201) |

16:40 - 17:20 |
Alexis Arnaudon: Geometric modelling of uncertainties ↓ In mechanics, and in particular in shape analysis, taking into account the underlying geometric properties of a problem to model it is often crucial to understand and solve it. This approach has mostly been applied for isolated systems, or for systems interacting with a well-defined, deterministic environment. In this talk, I want to discuss how to go beyond this deterministic description of isolated systems to include random interactions with an environment, while retaining as much as possible the geometric properties of the isolated systems. I will discuss examples from geometric mechanics to shape analysis, ranging from interacting rigid bodies with a heath bath to uncertainties quantification in computational anatomy. (TCPL 201) |

17:30 - 19:30 |
Dinner ↓ A buffet dinner is served daily between 5:30pm and 7:30pm in the Vistas Dining Room, the top floor of the Sally Borden Building. (Vistas Dining Room) |

Tuesday, December 11 | |
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07:00 - 09:00 | Breakfast (Vistas Dining Room) |

09:00 - 09:40 |
Bernhard Schmitzer: Semi-discrete unbalanced optimal transport and quantization ↓ "Semi-discrete optimal transport between a discrete source and a continuous target has intriguing geometric properties and applications in modelling and numerical methods. Unbalanced transport, which allows the comparison of measures with unequal mass, has recently been studied in great detail by various authors.
In this talk we consider the combination of both concepts. The tessellation structure of semi-discrete transport survives and there is an interplay between the length scales of the discrete source and unbalanced transport which leads to qualitatively new regimes in the crystallization limit." (TCPL 201) |

09:40 - 10:20 |
Carola-Bibiane Schönlieb: Wasserstein for learning image regularisers ↓ In this talk we will discuss the use of a Wasserstein loss function for learning regularisers in an adversarial manner. This talk is based on joint work with Sebastian Lunz and Ozan Öktem, see https://arxiv.org/abs/1805.11572 (TCPL 201) |

10:20 - 10:50 | Coffee Break (TCPL Foyer) |

10:50 - 11:30 | Tryphon Georgiou (TCPL 201) |

11:30 - 12:10 |
Laurent Younes: Normal coordinates and equivolumic layers estimation in the cortex (tentative) ↓ TBA (TCPL 201) |

12:10 - 14:00 | Lunch (Vistas Dining Room) |

14:00 - 16:30 | Discussion sessions (TCPL 201) |

15:00 - 15:30 | Coffee Break (TCPL Foyer) |

16:30 - 17:10 |
Barbara Gris: Analyze shape variability via deformations ↓ I will present how shape registration via constrained deformations can help understanding the variability within a population of shapes. (TCPL 201) |

17:10 - 17:50 |
Dongyang Kuang: Convnets, a different view of approximating diffeomorphisms in medical image registration ↓ As with the heat of artificial intelligence, there are more and more researches starting to investigate the possible geometric transformations using data-driven methods such as convolutional neural networks. In this talk, I will start by introducing some existing work that learn 2D linear transformations in an unsupervised way. This then will be followed by an overview of some recent works focusing on nonlinear transformations in 3D volumetric data. Finally, I will present results from the joint work with my supervisor using our network architecture called FAIM. (TCPL 201) |

17:50 - 19:30 | Dinner (Vistas Dining Room) |

Wednesday, December 12 | |
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07:00 - 09:00 | Breakfast (Vistas Dining Room) |

09:00 - 09:40 |
Stephen Preston: Solar models for Euler-Arnold equations ↓ Many one-dimensional Euler-Arnold equations can be recast in the form of a central-force problem $\Gamma_{tt}(t,x) = -F(t,x) \Gamma(t,x)$, where $\Gamma$ is a vector in $\mathbb{R}^2$ and $F$ is a nonlocal function possibly depending on $\Gamma$ and $\Gamma_t$. Angular momentum of this system is precisely the conserved momentum for the Euler-Arnold equation. In particular this picture works for the Camassa-Holm equation, the Hunter-Saxton equation, and the Okamoto-Sakajo-Wunsch family of equations.
In the solar model, breakdown comes from a particle hitting the origin in finite time, which is only possible with zero angular momentum. Results due to McKean (for Camassa-Holm), Lenells (for Hunter-Saxton), and Bauer-Kolev-Preston/Washabaugh (for the Wunsch equation) show that breakdown of smooth solutions occurs exactly when momentum changes from positive to negative. I will discuss some conjectures and numerical evidence for the generalization of this picture to other equations such as the $\mu$-Camassa-Holm equation or the DeGregorio equation. (TCPL 201) |

09:40 - 10:20 |
Cy Maor: Vanishing geodesic distance for right-invariant Sobolev metrics on diffeomorphism groups ↓ Since the seminal work of Arnold on the Euler equations, many important PDEs were shown to be geodesic equations of diffeomorphism groups of manifolds, with respect to various Sobolev norms. But what about the geodesic distance induced by these norms? Is it positive between different diffeomorphisms, or not?
In this talk I will show that the geodesic distance on the diffeomorphism group of an $n$-dimensional manifold, induced by the $W^{s,p}$ norm, does not vanish if and only if $s\ge 1$ or $sp>n$. The first condition detects changes of volume, while the second one detects transport of arbitrary small sets. I will focus on the case where both conditions fail, and how this enables the construction of arbitrary short paths between diffeomorphisms.
Based on a joint work with Robert Jerrard, following works of Michor-Mumford, Bauer-Bruveris-Harms-Michor and Bauer-Harms-Preston. (TCPL 201) |

10:20 - 10:50 | Coffee Break (TCPL Foyer) |

10:50 - 11:30 |
Philipp Harms: Smooth perturbations of the functional calculus and applications to Riemannian geometry on spaces of metrics ↓ We show that the functional calculus, which maps operators A to functionals f(A), is holomorphic for a certain class of operators A and holomorphic functions f. Using this result we are able to prove that fractional Laplacians depend real analytically on the underlying Riemannian metric in suitable Sobolev topologies.
As an application we obtain local well-posedness of the geodesic equation for fractional Sobolev metrics on the space of all Riemannian metrics. (Joint work with Martins Bruveris, Martin Bauer, and Peter W. Michor) (TCPL 201) |

11:30 - 12:10 |
Eric Klassen: Comparing Shapes of Curves, Surfaces, and Higher Dimensional Immersions in Euclidean Space ↓ Comparing shapes and treating them as data for statistical analyses has many applications in biology and elsewhere. Certain elastic metrics on spaces of immersions have proved very effective for comparing curves and surfaces. The elastic metrics which have proved most useful for computation have been first order metrics, i.e., they compare tangent vectors on the shapes rather than points on the shapes. In this talk I will present a unifying view of these metrics, shedding new light on old methods and, I hope, suggesting new methods for analyzing surfaces and higher dimensional shapes. (TCPL 201) |

12:10 - 13:30 | Lunch (Vistas Dining Room) |

13:30 - 17:30 | Free Afternoon (Banff National Park) |

17:30 - 19:30 | Dinner (Vistas Dining Room) |

Thursday, December 13 | |
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07:00 - 09:00 | Breakfast (Vistas Dining Room) |

09:00 - 09:40 |
Facundo Memoli: Metrics on the collection of dynamic shapes ↓ When studying flocking/swarming behaviors in animals one is interested in quantifying and comparing the dynamics of the clustering induced by the coalescence and disbanding of groups of animals. In a similar vein, when attempting to classify motion capture data according to action one is confronted with having to match/compare shapes that evolve with time.
Motivated by these applications, we study the question of suitably metrizing the collection of all dynamic metric spaces (DMSs). We construct a suitable metric on this collection and prove the stability of several natural invariants of DMSs under this metric. In particular, we prove that certain zigzag persistent homology invariants related to dynamic clustering are stable w.r.t. this distance.
These lower bounds permit the efficient classification of dynamic shape data in applications. We will show computational experiments on dynamic data generated via distributed behavioral models.
This is joint work with Woojin Kim and Zane Smith
https://research.math.osu.edu/networks/formigrams/ (TCPL 201) |

09:40 - 10:20 |
Tom Needham: Gromov-Monge Quasimetrics and Distance Distributions ↓ In applications in computer graphics and computational anatomy, one seeks a measure-preserving map from one shape to another which preserves geometry as much as possible. Inspired by this, we consider a notion of distance between arbitrary compact metric measure spaces by blending the Monge formulation of optimal transport with the Gromov-Hausdorff construction. We show that the resulting distance is an extended quasi-metric on the space of compact mm-spaces. This distance has convenient lower bounds defined in terms of distance distributions; these are functions associated to mm-spaces which have been used frequently as summaries in data and shape analysis applications. We provide rigorous results on the effectiveness of these lower bounds when restricted to simple classes of mm-spaces such as metric graphs or plane curves.This is joint work with Facundo Mémoli. (TCPL 201) |

10:20 - 10:50 | Coffee Break (TCPL Foyer) |

10:50 - 11:30 |
Jean-David Benamou: Dynamic formulations of optimal transportation and variational relaxation of Euler equations. ↓ We will briefly recall the classical Optimal Transportation Framework and its Dynamic relaxations. We will show the link between these Dynamic formulation and the so-called MultiMarginal extension of Optimal Transportation. We will then describe the so-called Iterative Proportional Fitting Procedure (aka Sinkhorn method) which can be efficiently applied to the multi-marginal OT setting. Finally we will show how this can be used to compute generalized Euler geodesics due to Brenier. This problem can be considered as the oldest instance of Multi-Marginal Optimal Transportation problem.
Joint work with Guillaume Carlier (Ceremade, Universite Paris Dauphine, France) and Luca Nenna (U. Paris Sud, France). (TCPL 201) |

11:30 - 12:10 | Tudor Ratiu: Group valued momentum maps (TCPL 201) |

12:10 - 14:00 | Lunch (Vistas Dining Room) |

14:00 - 14:20 | Andrea Natale (TCPL 201) |

14:20 - 14:40 | Jean Feydy (TCPL 201) |

14:40 - 15:00 | Alice Le Brigant: Quantization on a Riemannian manifold with application to air traffic control (TCPL 201) |

15:00 - 15:30 | Coffee Break (TCPL Foyer) |

17:30 - 19:30 | Dinner (Vistas Dining Room) |

Friday, December 14 | |
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07:00 - 09:00 | Breakfast (Vistas Dining Room) |

09:00 - 09:40 |
Martin Rumpf: Spline curves in Riemannian shape spaces ↓ We investigate a generalization of cubic splines to Riemannian manifolds.
Spline curves are defined as minimizers of the spline energy---a combination of the Riemannian path energy and
the time integral of the squared covariant derivative of the path velocity---under suitable interpolation conditions.
A variational time discretization for the spline energy leads to a constrained optimization problem over discrete paths on the manifold.
Existence of continuous and discrete spline curves is established using the direct method in the calculus of variations.
Furthermore, the convergence of discrete spline paths to a continuous spline curve
follows from the $\Gamma$-convergence of the discrete to the continuous spline energy.
Finally, selected example settings are discussed, including splines on embedded finite-dimensional manifolds,
on a high-dimensional manifold of discrete shells with applications in surface processing,
and on the infinite-dimensional shape manifold of viscous rods. (TCPL 201) |

09:40 - 10:20 |
Nicolas Charon: Diffeomorphic matching and simplification of geometric distributions. ↓ This talk will introduce deformation models on spaces of oriented varifolds within the LDDMM framework as well as a general registration algorithm for those objects which embeds many previously considered geometric structures like curves, surfaces but also orientation distribution fields... We will also discuss the issue of compressing/quantizing oriented varifold representations in order to numerically accelerate diffeomorphic registration procedures. (TCPL 201) |

10:20 - 10:50 | Coffee Break (TCPL Foyer) |

11:30 - 12:00 |
Checkout by Noon ↓ 5-day workshop participants are welcome to use BIRS facilities (BIRS Coffee Lounge, TCPL and Reading Room) until 3 pm on Friday, although participants are still required to checkout of the guest rooms by 12 noon. (Front Desk - Professional Development Centre) |

12:00 - 13:30 | Lunch from 11:30 to 13:30 (Vistas Dining Room) |