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Ugo Bessi

Publications and source records attributed to Ugo Bessi.

16 recordsLinked to original sources
arXiv2026

Harmonic immersions of the Sierpinski gasket into the hyperbolic plane

Many fractals $G$ admit a harmonic immersion into $\R^n$, i.e. an immersion which minimises a natural energy under fixed boundary conditions; we look for harmonic immersions of the Sierpinski gasket into the hyperbolic plane. We show that, given any three points $\tilde A$, $\tilde B$, $\tilde C$ in the hyperbolic plane there is a harmonic map bringing the three points $A$, $B$, $C$ of the boundary of the gasket to $\tilde A$, $\tilde B$, $\tilde C$ respectively. Moreover, if the points $\tilde A$, $\tilde B$, $\tilde C$ are sufficiently close in the hyperbolic distance, then the harmonic map is unique and depends differentiably on $\tilde A$, $\tilde B$, $\tilde C$. Lastly, we show that, if the harmonic map $\phi$ is injective, then it brings geodesics of the gasket $G$ into geodesics of $\phi(G)$.

math.AP↗
arXiv2023

Harmonic embeddings of the stretched Siepinski gasket

P. Alonso-Ruiz, U. Freiberg and J. Kigami have defined a large family of resistance forms on the Stretched Sierpinski Gasket $G$. In the present paper we introduce a system of coordinates on $G$ (technically, an embedding of $G$ into $\R^2$) such that \noindent$\bullet$) these forms are defined on $C^1(\R^2,\R)$ and \noindent$\bullet$) all affine functions are harmonic for them. We do this adapting a standard method from the Harmonic Sierpinski Gasket: we start finding a sequence $G_l$ of pre-fractals such that all affine functions are harmonic on $G_l$. After showing that this property is inherited by the stretched harmonic gasket $G$, we use the formula for the Laplacian of a composition to prove that, for a natural measure $μ$ on $G$, $C^2(\R^2,\R)\subset\dc(Δ)$ and Teplyaev's formula for the Laplacian of $C^2$ functions holds. Lastly, we use the expression for $Δu$ to show that the form we have found is closable in $L^2(G,μ)$.

math.MG↗
arXiv2023

Counting periodic orbits on fractals weighted by their Lyapunov exponents

Several authors have shown that Kusuoka's measure $κ$ on fractals is a scalar Gibbs measure; in particular, it maximises a pressure. There is also a different approach, in which one defines a matrix-valued Gibbs measure $μ$ which induces both Kusuoka's measure $κ$ and Kusuoka's bilinear form. In the first part of the paper we show that one can define a "pressure" for matrix valued measures; this pressure is maximised by $μ$. In the second part, we use the matrix-valued Gibbs measure $μ$ to count periodic orbits on fractals, weighted by their Lyapounov exponents.

math.DS↗
arXiv2020

Cheeger's energy on the Harmonic Sierpinski Gasket

Koskela and Zhou have proven that, on the harmonic Sierpinski gasket with Kusuoka's measure, the "natural" Dirichlet form coincides with Cheeger's energy. We give a different proof of this result, which uses the properties of the Lyapounov exponent of the gasket.

math.MG↗
arXiv2020

Another point of view on Kusuoka's measure

Kusuoka's measure on fractals is a Gibbs measure of a very special kind, because its potential is discontinuous, while the standard theory of Gibbs measures requires continuous (actuallly, Hölder) potentials. In this paper, we shall see that for many fractals it is possible to build a class of matrix-valued Gibbs measures completely within the scope of the standard theory; there are naturally some minor modifications, but they are only due to the fact that we are dealing with matrix-valued functions and measures. We shall use these matrix-valued Gibbs measures to build self-similar Dirichlet forms on fractals. Moreover, we shall see that Kusuoka's measure can be recovered in a simple way from the matrix-valued Gibbs measure.

math.MG↗
arXiv2020

Hamilton-Jacobi in metric spaces with a homological term

The Hamilton-Jacobi equation on metric spaces has been studied by several authors; following the approach of Gangbo and Swiech, we show that the final value problem for the Hamilton-Jacobi equation has a unique solution even if we add a homological term to the Hamiltonian. In metric measure spaces which satisfy the $RCD(K,\infty)$ condition one can define a Laplacian which shares many properties with the ordinary Laplacian on $\R^n$; in particular, it is possible to formulate a viscous Hamilton-Jacobi equation. We show that, if the homological term is sufficiently regular, the viscous Hamilton-Jacobi equation has a unique solution also in this case.

math.OC↗
arXiv2018

An entropy generation formula on $RCD(K,\infty)$ spaces

J. Feng and T. Nguyen have shown that the solutions of the Fokker-Planck equation in $\R^d$ satisfy an entropy generation formula. We prove that, in compact metric measure spaces with the $RCD(K,\infty)$ property, a similar result holds for curves of measures whose density is bounded away from zero and infinity. We use this fact to show the existence of minimal characteristics for the stochastic value function.

math.AP↗
arXiv2016

A cost on paths of measures which induces the Fokker-Planck equation

J. Feng and T. Nguyen have defined a cost on curves of measures which is finite exactly on the curves which solve a Fokker-Planck equation with $L^2$ drift. In this paper, using ideas of D. Gomes and E. Valdinoci, we give a different construction of the cost of Feng and Nguyen.

math.OC↗
arXiv2016

The stochastic value function on metric measure spaces

Let $(S,d)$ be a compact metric space and let $m$ be a Borel probability measure on $(S,d)$. We shall prove that, if $(S,d,m)$ is a $RCD(K,\infty)$ space, then the stochastic value function satisfies the viscous Hamilton-Jacobi equation, exactly as in Fleming's theorem on ${\bf R}^d$.

math.AP↗
arXiv2016

A time-step approximation scheme for a viscous version of the Vlasov equation

Gomes and Valdinoci have introduced a time-step approximation scheme for a viscous version of Aubry-Mather theory; this scheme is a variant of that of Jordan, Kinderlehrer and Otto. Gangbo and Tudorascu have shown that the Vlasov equation can be seen as an extension of Aubry-Mather theory, in which the configuration space is the space of probability measures, i. e. the different distributions of infinitely many particles on a manifold. Putting the two things together, we show that Gomes and Valdinoci's theorem carries over to a viscous version of the Vlasov equation. In this way, we shall recover a theorem of J. Feng and T. Nguyen, but by a different and more "elementary" proof.

math.AP↗
arXiv2016

Chaotic motions for a version of the Vlasov equation

We consider a version of the Vlasov equation on the circle under a periodic potential $V(x,t)$ and a repulsing smooth interaction $W$. We suppose that the Lagrangian for the single particle has chaotic orbits; using Aubry-Mather theory and ideas of W. Gangbo, A. Tudorascu and P. Bernard, we prove that, for any initial distribution of particles, it is possible to choose their initial speed in such a way to get a chaotic orbit on $[0,+\infty)$.

math.AP↗
arXiv2016

The Aubry set for a version of the Vlasov equation

We check that several properties of the Aubry set, first proven for finite-dimensional Lagrangians by Mather and Fathi, continue to hold in the case of the infinitely many interacting particles of the Vlasov equation on the circle.

math.AP↗
arXiv2016

Viscous Aubry-Mather theory and the Vlasov equation

The Vlasov equation models a group of particles moving under a potential $V$; moreover, each particle exerts a force, of potential $W$, on the other ones. We shall suppose that these particles move on the $p$-dimensional torus ${\bf T}^p$ and that the interaction potential $W$ is smooth. We are going to perturb this equation by a Brownian motion on ${\bf T}^p$; adapting to the viscous case methods of Gangbo, Nguyen, Tudorascu and Gomes, we study the existence of periodic solutions and the asymptotics of the Hopf-Lax semigroup.

math.AP↗
arXiv2010

Young measures, Cartesian maps, and polyconvexity

We consider the variational problem consisting of minimizing a polyconvex integrand for maps between manifolds. We offer a simple and direct proof of the existence of a minimizing map. The proof is based on Young measures.

math.OC↗