Adjoint divisors on algebraic curves

Let C be a numerically connected curve lying on a smooth algebraic surface. We show that if is an ample invertible sheaf satisfying some technical numerical hypotheses then is normally generated. As a corollary we show that the sheaf ω_C^⊗2 on a numerically connected curve C of arithmetic genus p_a?3 is normally generated if ω_C is ample and does not exist a subcurve B⊂C such that p_a(B)=1=B(C−B).     

The algebraic independence of the sum of divisors functions

Let σj(n)=_∑d|ndj be the sum of divisors function, and let I be the identity function. When considering only one input variable n, we show that the set of functions is algebraically independent. With two input variables, we give a non-trivial identity involving the sum of divisors function, prove its uniqueness, and use it to prove that any perfect number n must have the form n=rσ(r)/(2r−σ(r)), with some restrictions on r. This generalizes the known forms for both even and odd perfect numbers.     

On algebraic integrability of Gelfand-Zeitlin fields

We generalize a result of Kostant and Wallach concerning the algebraic integrability of the Gelfand-Zeitlin vector fields to the full set of strongly regular elements in \mathfrakg\mathfrakl \mathfrak{g}\mathfrak{l} (n, ℂ). We use decomposition classes to stratify the strongly regular set by subvarieties X_D {X_\mathcal{D}} . We construct an étale cover [^(\mathfrakg)]_D {\hat{\mathfrak{g}}}_\mathcal{D} of X_D {X_\mathcal{D}} and show that X_D {X_\mathcal{D}} and [^(\mathfrakg)]_D {\hat{\mathfrak{g}}}_\mathcal{D} are smooth and irreducible. We then use Poisson geometry to lift the Gelfand-Zeitlin vector fields on X_D {X_\mathcal{D}} to Hamiltonian vector fields on [^(\mathfrakg)]_D {\hat{\mathfrak{g}}}_\mathcal{D} and integrate these vector fields to an action of a connected, commutative algebraic group.     

Dicritical divisors and Jacobian problem

The dicritical divisor coming out of integrable holonomic systems gets miraculously married to the jacobian problem of algebraic geometry.     

A general class of centers for the Poincaré problem

We consider the classical Poincaré problem of a linear center perturbed by homogeneous polynomials of degree n?2. Using certain parity properties of a related differential equation, we develop a technique for obtaining center conditions for an arbitrary value of n and use it to exhibit explicitly new center conditions for n=4,…,8.     

Poincaré invariants

We construct an obstruction theory for relative Hilbert schemes in the sense of [K. Behrend, B. Fantechi, The intrinsic normal cone, Invent. Math. 128 (1) (1997) 45-88] and compute it explicitly for relative Hilbert schemes of divisors on smooth projective varieties. In the special case of curves on a surface V, our obstruction theory determines a virtual fundamental class , which we use to define Poincaré invariants

     

Poincaré-Perron's method for the Dirichlet problem on stratified sets

We extend standard Poincaré-Perron's method to the Dirichlet problem on a class of multistructures. This method is based on the spherical mean theorem, the construction of fundamental solutions and on Harnack's inequality on such domains, that we first establish.     

The three-magnon problem and integrability of rung-dimerized spin ladders

We study the integrability problem for rung-dimerized spin ladder by the Bethe ansatz in three-magnon sector. It is shown that solvability of the three-magnon problem takes place for the same values of coupling constants in the Hamiltonian which guarantee solvability of the Yang–Baxter equation for the corresponding R-matrix. Bibliography: 15 titles.     

Equivariant completions of homogenous algebraic varieties by homogenous divisors

Complete smooth complex algebraic varieties with an almost transitive action of a linear algebraic group are studied. They are classified in the case, when the complement of the open orbit is a homogeneous hypersurface. If the group and the isotropy subgroup at a generic point are both reductive, then there exists a natural one-to-one correspondence between these two-orbit varieties and compact riemannian symmetric spaces of rank one.     

Hamiltonicity and integrability of the Suslov problem

The Hamiltonian representation and integrability of the nonholonomic Suslov problem and its generalization suggested by S. A. Chaplygin are considered. This subject is important for understanding the qualitative features of the dynamics of this system, being in particular related to a nontrivial asymptotic behavior (i. e., to a certain scattering problem). A general approach based on studying a hierarchy in the dynamical behavior of nonholonomic systems is developed.     

Non-special divisors on real algebraic curves and embeddings into real projective spaces

We show that there is a large class of non-special divisors of relatively small degree on a given real algebraic curve. If the real algebraic curve has many real components, such a divisor gives rise to an embedding (birational embedding, resp.) of the real algebraic curve into the real projective space ℙ ^r for r≥3 (r=2, resp.). We study these embeddings in quite some detail. Received: October 17, 2001?Published online: February 20, 2003     

Rotation numbers, eigenvalues, and the Poincaré-Birkhoff theorem

Using the relation between rotation numbers and eigenvalues, we prove the existence of nontrivial T-periodic solutions having precise oscillatory properties for a class of asymptotically linear second order differential equations.     

Equivalence problem and integrability of the Riccati equations

We consider the class of general real Riccati equations and find its Lie group of equivalence transformations. Using the Lie algebra of this Lie group and its invariants we formulate criteria of equivalence of the Riccati equations. These criteria determine some cases of the general Riccati equations, which are integrable in quadratures.     

New Poincaré-type inequalities

We present some Poincaré-type inequalities for quadratic matrix fields with applications e.g. in gradient plasticity or fluid dynamics. In particular, applications to the pseudostress–velocity formulation of the stationary Stokes problem and to infinitesimal gradient plasticity are discussed.