We prove that the number of critical points of a Li–Tam Green’s function on a complete open Riemannian surface of finite type admits a topological upper bound, given by the first Betti number of the surface. In higher dimensions, we show that there are no topological upper bounds on the number of critical points by constructing, for each nonnegative integer $N$, a Riemannian manifold diffeomorphic to $\mathbb{R}^n\: (n \ge 3)$ whose minimal Green’s function has at least $N$ non-degenerate critical points. Variations on the method of proof of the latter result yield contractible $n$-manifolds whose minimal Green’s functions have level sets diffeomorphic to any fixed codimension 1 compact submanifold of $\mathbb{R}^n$.

For a path in a compact finite dimensional Alexandrov space $X$ with curv $\ge \kappa$, the two basic geometric invariants are the length and the turning angle (which measures the closeness from being a geodesic). We show that the sum of the two invariants of any loop is bounded from below in terms of $\kappa$, the dimension, diameter, and Hausdorff measure of $X$. This generalizes a basic estimate of Cheeger on the length of a closed geodesic in a closed Riemannian manifold. To see that the above result also generalizes and improves an analog of the Cheeger type estimate in Alexandrov geometry in "A.D. Alexandrov spaces with curvature bounded below," we show that for a class of subsets of $X$, the $n$-dimensional Hausdorff measure and rough volume are proportional by a constant depending on $n = \dim(X)$.

We obtain a growth estimate for the number of lattice points inside any $\mathbb{Q}$-Gorenstein cone. Our proof uses the result of Futaki-Ono-Wang on Sasaki-Einstein metric for the toric Sasakian manifold associated to the cone, a Yau’s inequality, and the Kawasaki-Riemann-Roch formula for orbifolds.

Let $U$ be a connected non-singular quasi-projective variety and $f : A \to U$ a family of abelian varieties of dimension $g$. Suppose that the induced map $U \to \mathcal{A}_g$ is generically finite and there is a compactification $Y$ with complement $S = Y \backslash U$ a normal crossing divisor such that $\Omega_Y^1 (\log S)$ is nef and $\omega_Y (S)$ is ample with respect to $U.

We characterize whether $U$ is a Shimura variety by numerical data attached to the variation of Hodge structures, rather than by properties of the map $U \to \mathcal{A}_g$ or by the existence of CM points.

More precisely, we show that $f : A \to U$ is a Kuga fibre space, if and only if two conditions hold. First, each irreducible local subsystem $\mathbb{V}$ of $R_1 f_* \mathbb{C}_A$ is either unitary or satisfies the Arakelov equality. Second, for each factor $M$ in the universal cover of $U$ whose tangent bundle behaves like that of a complex ball, an iterated Kodaira-Spencer map associated with $V$ has minimal possible length in the direction of $M$. If in addition $f : A \to U$ is rigid, it is a connected Shimura subvariety of $\mathcal{A}_g$ of Hodge type.

We define a hierarchy of Hamiltonian PDEs associated to an arbitrary tau-function in the semi-simple orbit of the Givental group action on genus expansions of Frobenius manifolds. We prove that the equations, the Hamiltonians, and the bracket are weighted-homogeneous polynomials in the derivatives of the dependent variables with respect to the space variable.

In the particular case of a conformal (homogeneous) Frobenius structure, our hierarchy coincides with the Dubrovin–Zhang hierarchy that is canonically associated to the underlying Frobenius structure. Therefore, our approach allows to prove the polynomiality of the equations, Hamiltonians, and one of the Poisson brackets of these hierarchies, as conjectured by Dubrovin and Zhang.