Homepage of Dario Mazzoleni
Main research topics

Existence and surgery results
Existence of minimizers for spectral problems
One of my main research interest is the study of existence of solutions for spectral shape optimization problems in an unbounded setting. The main tool for proving existence in the bounded case is a wellknown Theorem by Buttazzo and Dal Maso and it is not easy to extend it in the general case.
In this paper (with Aldo Pratelli) we show that any increasing functional of the first k eigenvalues of the Dirichlet Laplacian admits a (quasi)open minimizer among the subsets of R^N of unit measure. In particular, there exists such a minimizer which is bounded, where the bound depends on k and N, but not on the functional.
Moreover, in the paper Boundedness of minimizers for spectral problems in R^N, I also proved that all minimizers for such a problem have diameter uniformly bounded.
The techniques developed in these works consists, roughly speaking, in taking an open set and show that if it has 'long tails' it is better to cut them (and slightly rearrange the set) so that, after rescaling to unit measure, the first k eigenvalues are decreased.
One of my main research interest is the study of existence of solutions for spectral shape optimization problems in an unbounded setting. The main tool for proving existence in the bounded case is a wellknown Theorem by Buttazzo and Dal Maso and it is not easy to extend it in the general case.
In this paper (with Aldo Pratelli) we show that any increasing functional of the first k eigenvalues of the Dirichlet Laplacian admits a (quasi)open minimizer among the subsets of R^N of unit measure. In particular, there exists such a minimizer which is bounded, where the bound depends on k and N, but not on the functional.
Moreover, in the paper Boundedness of minimizers for spectral problems in R^N, I also proved that all minimizers for such a problem have diameter uniformly bounded.
The techniques developed in these works consists, roughly speaking, in taking an open set and show that if it has 'long tails' it is better to cut them (and slightly rearrange the set) so that, after rescaling to unit measure, the first k eigenvalues are decreased.
This main idea can be slightly improved, as we did (with Dorin Bucur) in the paper A surgery result for the spectrum of the DirichletLaplacian.
More precisely, we give a method to geometrically modify an open set such that the first k eigenvalues of the Dirichlet Laplacian and its perimeter are not increasing, its measure remains constant, and both perimeter and diameter decrease below a certain threshold. The key point of the analysis relies on the properties of the shape subsolutions for the torsion energy and on a new version of AltCaffarelli ideas for regularity of free boundaries.
For a survey on these topics, see Recent existence results for spectral problems.
More precisely, we give a method to geometrically modify an open set such that the first k eigenvalues of the Dirichlet Laplacian and its perimeter are not increasing, its measure remains constant, and both perimeter and diameter decrease below a certain threshold. The key point of the analysis relies on the properties of the shape subsolutions for the torsion energy and on a new version of AltCaffarelli ideas for regularity of free boundaries.
For a survey on these topics, see Recent existence results for spectral problems.
Regularity results
Lipschitz regularity of the eigenfunctions on optimal domains
The above mentioned existence results provide an optimal set in the class of 'quasiopen' sets. This is a very big class, hence a natural question is whether optimal set are more regular (at least open).
In this work with Dorin Bucur, Aldo Pratelli and Bozhidar Velichkov, we study the optimal sets Ω⊂R^N for spectral functionals F(λ_1(A),…,λ_p(A)), which are biLipschitz with respect to each of the eigenvalues λ_1(A),…,λ_p(A) of the Dirichlet Laplacian on A, a prototype being the problem
min{λ_1(A)+⋯+λ_p(A) : A⊂R^N, A=1}.
We prove the Lipschitz regularity of the eigenfunctions u_1,…,u_p of the Dirichlet Laplacian on the optimal set Ω and, as a corollary, we deduce that Ω is open.
For functionals depending only on a generic subset of the spectrum, as for example λ_k(A), our result proves only the existence of a Lipschitz continuous eigenfunction in correspondence to each of the eigenvalues involved.
The techniques used are based on a free boundary approach (employing ideas from AltCaffarelli), combined with the careful study of shape supersolutions, i.e. sets which are optimal with respect to external perturbations.
$C^(\infty)$ regularity for optimal sets
In the paper Regularity of the optimal sets for some spectral functionals with S. Terracini and B. Velichkov,we investigate the regularity issue for the functional λ_1+⋯+λ_p.
We prove an analogous result to the theorems proved by Alt, Caffarelli and Weiss for the regularity of the onephase free boundary, that is, showing that every optimal set Ω has topological boundary made by a regular part which is $C^{\infty}$ regular, and a singular part, which is empty if d<d∗, contains at most a finite number of isolated points if d=d∗ and has Hausdorff dimension smaller than (d−d∗) if d>d∗, where the natural number d∗∈[5,7] is the smallest dimension at which minimizing onephase free boundaries admit singularities.
The main difference is that we have a vectorial setting, so we have to deal with functions that do not have constant sign and to treat the optimality condition in an appropriate way.
The main steps of our proof are the following:
In another work with S. Terracini and B. Velichkov, Regularity of the free boundary for the vectorial Bernoulli problem, we study the regularity for the free bondary of a vectorvalued Bernoulli problem with no sign assumption on any of the components. This is strongly related to the regularity of optimal shapes for the kth eigenvlaue.
We can prove that the free boundary is made by a regular part, which is locally the graph of a smooth function, a singular part which is of lower Hausdorff dimension and a set of branching points, which has finite d1 Hausdorff measure.
For this purpose we shall exploit the NTA property of the regular part to reduce ourselves to a scalar onephase Bernoulli problem, to which we can apply previous results.
The main issue that we are now approaching is to prove that the set of branching points is contained in a d2 dimensional regular submanifold. This is a very hard topic as there are currently very few results which give a stratification of cusps and branching points.
The above mentioned existence results provide an optimal set in the class of 'quasiopen' sets. This is a very big class, hence a natural question is whether optimal set are more regular (at least open).
In this work with Dorin Bucur, Aldo Pratelli and Bozhidar Velichkov, we study the optimal sets Ω⊂R^N for spectral functionals F(λ_1(A),…,λ_p(A)), which are biLipschitz with respect to each of the eigenvalues λ_1(A),…,λ_p(A) of the Dirichlet Laplacian on A, a prototype being the problem
min{λ_1(A)+⋯+λ_p(A) : A⊂R^N, A=1}.
We prove the Lipschitz regularity of the eigenfunctions u_1,…,u_p of the Dirichlet Laplacian on the optimal set Ω and, as a corollary, we deduce that Ω is open.
For functionals depending only on a generic subset of the spectrum, as for example λ_k(A), our result proves only the existence of a Lipschitz continuous eigenfunction in correspondence to each of the eigenvalues involved.
The techniques used are based on a free boundary approach (employing ideas from AltCaffarelli), combined with the careful study of shape supersolutions, i.e. sets which are optimal with respect to external perturbations.
$C^(\infty)$ regularity for optimal sets
In the paper Regularity of the optimal sets for some spectral functionals with S. Terracini and B. Velichkov,we investigate the regularity issue for the functional λ_1+⋯+λ_p.
We prove an analogous result to the theorems proved by Alt, Caffarelli and Weiss for the regularity of the onephase free boundary, that is, showing that every optimal set Ω has topological boundary made by a regular part which is $C^{\infty}$ regular, and a singular part, which is empty if d<d∗, contains at most a finite number of isolated points if d=d∗ and has Hausdorff dimension smaller than (d−d∗) if d>d∗, where the natural number d∗∈[5,7] is the smallest dimension at which minimizing onephase free boundaries admit singularities.
The main difference is that we have a vectorial setting, so we have to deal with functions that do not have constant sign and to treat the optimality condition in an appropriate way.
The main steps of our proof are the following:
 We prove a nondegeneracy lemma for the vector $U=(u_1,\dots, u_k)$ and then show that actually if $U$ is nondegenerate, than $u_1$ is nondegenerate, too. Moreover we give some density estimates.
 We give a monotonicity formula (in the spirit of Weiss) and make a blowup analysis, showing that the blowup limits are optimal and $1$homogeneous.
 We study the optimality condition in the viscosity sense, we identify the regular and singular part of the topological boundary and then we reduce ourselves to a problem with only one positive function and apply the regularity result for the classical AltCaffarelli free boundary problem.
At the end we provide the estimates on the Hausdorff dimension for the singular part of the boundary.
In another work with S. Terracini and B. Velichkov, Regularity of the free boundary for the vectorial Bernoulli problem, we study the regularity for the free bondary of a vectorvalued Bernoulli problem with no sign assumption on any of the components. This is strongly related to the regularity of optimal shapes for the kth eigenvlaue.
We can prove that the free boundary is made by a regular part, which is locally the graph of a smooth function, a singular part which is of lower Hausdorff dimension and a set of branching points, which has finite d1 Hausdorff measure.
For this purpose we shall exploit the NTA property of the regular part to reduce ourselves to a scalar onephase Bernoulli problem, to which we can apply previous results.
The main issue that we are now approaching is to prove that the set of branching points is contained in a d2 dimensional regular submanifold. This is a very hard topic as there are currently very few results which give a stratification of cusps and branching points.
Geometric properties of optimal sets
In the paper (with Mette Iversen) Minimising convex combinations of low eigenvalues, we studied whether optimal sets for convex combinations of the first three eigenvalues are connected.
Moreover it is interesting to study geometric properties of optimal sets for functionals involving the first two eigenvalues: convexity, regularity...
With Davide Zucco (Convex combinations of low eigenvalues, Fraenkel asymmetries and attainable sets) we used a new technique for proving nonconvexity of minimizers for convex combinations of the two lowest eigenvalues. The main point is to employ quantitative inequalities for the second Dirichlet eigenvalue and the study of the optimal convex set for the Fraenkel 2asymmetry, which turns out to be a "mobilelike" set.
Moreover in a paper with Aldo Pratelli (Some estimates for the higher eigenvalues of sets close to the ball) we prove quantitative estimates for the higher eigenvalues of sets with λ_1 not eccessively large, of the following form:
$λ_k(\Omega)\la_k(B)\leq C(λ_1(\Omega)\la_1(B))^α$.
Moreover it is interesting to study geometric properties of optimal sets for functionals involving the first two eigenvalues: convexity, regularity...
With Davide Zucco (Convex combinations of low eigenvalues, Fraenkel asymmetries and attainable sets) we used a new technique for proving nonconvexity of minimizers for convex combinations of the two lowest eigenvalues. The main point is to employ quantitative inequalities for the second Dirichlet eigenvalue and the study of the optimal convex set for the Fraenkel 2asymmetry, which turns out to be a "mobilelike" set.
Moreover in a paper with Aldo Pratelli (Some estimates for the higher eigenvalues of sets close to the ball) we prove quantitative estimates for the higher eigenvalues of sets with λ_1 not eccessively large, of the following form:
$λ_k(\Omega)\la_k(B)\leq C(λ_1(\Omega)\la_1(B))^α$.
Some ongoing stuff
 Gradient flows in shape optimization (mainly for eigenvalues of potentials)
 Shape optimization problems for eigenvalues of p and s Laplacian
 Shape optimization and critical points
 Properties of minimizers of principal eigenvalue with indefinite weight and Neumann boundary conditions
 Quantitative estimates for eigenvalues with new asymmetries