### Working paper

## Degenerating sequences of conformal classes and the conformal Steklov spectrum

Let *M* be a closed smooth manifold. In 1999, Friedlander and Nadirashvili introduced a new differential invariant *𝐼*1(*𝑀*) using the first normalized nonzero eigenvalue of the Lalpace–Beltrami operator Δ*𝑔* of a Riemannian metric *g*. They defined it taking the supremum of this quantity over all Riemannian metrics in each conformal class, and then taking the infimum over all conformal classes. By analogy we use *k*-th eigenvalues of Δ*𝑔* to define the invariants *𝐼**𝑘*(*𝑀*) indexed by positive integers *k*. In the present paper the values of these invariants on surfaces are investigated. We show that *𝐼**𝑘*(*𝑀*)=*𝐼**𝑘*(𝕊2) unless *M* is a non-orientable surface of even genus. For orientable surfaces and *𝑘*=1 this was earlier shown by Petrides. In fact Friedlander and Nadirashvili suggested that *𝐼*1(*𝑀*)=*𝐼*1(𝕊2) for any surface *M* different from Rℙ2. We show that, surprisingly enough, this is not true for non-orientable surfaces of even genus, for such surfaces one has *𝐼**𝑘*(*𝑀*)>*𝐼**𝑘*(𝕊2). We also discuss the connection between the Friedlander–Nadirashvili invariants and the theory of cobordisms, and conjecture that *𝐼**𝑘*(*𝑀*) is a cobordism invariant.

In what follows the problem of finding upper bounds on denial probability and probability of erroneous decoding in a coded Dynamic Hopset Allocation Frequency Hopping system with noncoherent threshold reception is considered.

In this work we summarize some recent results to be included in a forthcoming paper [Bartoli, D., A. A. Davydov, A. A. Kreshchuk, S. Marcugini and F. Pambianco, Small complete caps in PG(3,q) and PG(4,q), preprint]. We present and analyze computational results concerning small complete caps in the projective spaces PG(N,q) of dimension N=3and N=4 over the finite field of order q. The results have been obtained using randomized greedy algorithms and the algorithm with fixed order of points (FOP). The new complete caps are the smallest known. Based on them, we obtained new upper bounds on the minimum size t2(N,q) of a complete cap in PG(N,q), N=3,4. Our investigations and results allow to conjecture that these bounds hold for all q.

A model for organizing cargo transportation between two node stations connected by a railway line which contains a certain number of intermediate stations is considered. The movement of cargo is in one direction. Such a situation may occur, for example, if one of the node stations is located in a region which produce raw material for manufacturing industry located in another region, and there is another node station. The organization of freight traﬃc is performed by means of a number of technologies. These technologies determine the rules for taking on cargo at the initial node station, the rules of interaction between neighboring stations, as well as the rule of distribution of cargo to the ﬁnal node stations. The process of cargo transportation is followed by the set rule of control. For such a model, one must determine possible modes of cargo transportation and describe their properties. This model is described by a ﬁnite-dimensional system of diﬀerential equations with nonlocal linear restrictions. The class of the solution satisfying nonlocal linear restrictions is extremely narrow. It results in the need for the “correct” extension of solutions of a system of diﬀerential equations to a class of quasi-solutions having the distinctive feature of gaps in a countable number of points. It was possible numerically using the Runge–Kutta method of the fourth order to build these quasi-solutions and determine their rate of growth. Let us note that in the technical plan the main complexity consisted in obtaining quasi-solutions satisfying the nonlocal linear restrictions. Furthermore, we investigated the dependence of quasi-solutions and, in particular, sizes of gaps (jumps) of solutions on a number of parameters of the model characterizing a rule of control, technologies for transportation of cargo and intensity of giving of cargo on a node station.

Let k be a field of characteristic zero, let G be a connected reductive algebraic group over k and let g be its Lie algebra. Let k(G), respectively, k(g), be the field of k- rational functions on G, respectively, g. The conjugation action of G on itself induces the adjoint action of G on g. We investigate the question whether or not the field extensions k(G)/k(G)^G and k(g)/k(g)^G are purely transcendental. We show that the answer is the same for k(G)/k(G)^G and k(g)/k(g)^G, and reduce the problem to the case where G is simple. For simple groups we show that the answer is positive if G is split of type A_n or C_n, and negative for groups of other types, except possibly G_2. A key ingredient in the proof of the negative result is a recent formula for the unramified Brauer group of a homogeneous space with connected stabilizers. As a byproduct of our investigation we give an affirmative answer to a question of Grothendieck about the existence of a rational section of the categorical quotient morphism for the conjugating action of G on itself.

Let G be a connected semisimple algebraic group over an algebraically closed field k. In 1965 Steinberg proved that if G is simply connected, then in G there exists a closed irreducible cross-section of the set of closures of regular conjugacy classes. We prove that in arbitrary G such a cross-section exists if and only if the universal covering isogeny Ĝ → G is bijective; this answers Grothendieck's question cited in the epigraph. In particular, for char k = 0, the converse to Steinberg's theorem holds. The existence of a cross-section in G implies, at least for char k = 0, that the algebra k[G]G of class functions on G is generated by rk G elements. We describe, for arbitrary G, a minimal generating set of k[G]G and that of the representation ring of G and answer two Grothendieck's questions on constructing generating sets of k[G]G. We prove the existence of a rational (i.e., local) section of the quotient morphism for arbitrary G and the existence of a rational cross-section in G (for char k = 0, this has been proved earlier); this answers the other question cited in the epigraph. We also prove that the existence of a rational section is equivalent to the existence of a rational W-equivariant map T- - - >G/T where T is a maximal torus of G and W the Weyl group.