Vol 103, No 1-2 (2018)

The maximally monotone operator method in real weighted Lebesgue spaces is used to study three different classes of nonlinear singular integro-differential equations with an arbitrary positive parameter. Under sufficiently clear constraints on the nonlinearity, we prove existence and uniqueness theorems for the solution covering in particular, the linear case as well. In contrast to the previous papers in which other classes of nonlinear singular integral and integro-differential equations were studied, our study is based on the inversion of the superposition operator generating the nonlinearities of the equations under consideration and the establishment of the coercitivity of the inverse operator, as well as a generalization of the well-known Schleiff inequality.

Extremal problems on the number of j-independent sets in uniform simple hypergraphs are studied. Nearly optimal results on the maximum number of independent sets for the class of simple regular hypergraphs and on the minimum number of independent sets for the class of simple hypergraphs with given average degree of vertices are obtained.

Several sufficient conditions for product space to be strongly paracompact are given.

Rotation of a neutron in the coat of helium-5 as a classical particle for a relatively large value of the hidden parameter (measurement time) t_meas = h/E_ms is considered. In consideration of the asymptotics as N → 0, equations for the mesoscopic energy E_ms are given. A model for the helium nucleus is introduced and the values of the mesoscopic parameters M_ms, and E_ms for helium-4 are calculated.

The Elenbaas problem of electric discharge origination is considered. The mathematical model is an elliptic boundary-value problem with a parameter and discontinuous nonlinearity. The nontrivial solutions of the problem determine the free boundaries separating different phase states. A survey of results obtained for this problem is given. The greatest lower bound λmin of the values of the parameter λ for which the electric discharge is possible is obtained. The fact that the discharge domain appears for any λ ≥ λ_min is proved. The range of the parameter values for which the boundary of the discharge domain is of two-dimensional Lebesgue measure zero is determined. An unsolved problem is formulated.

It is well known that Givental’s toric Landau–Ginzburg models for Fano complete intersections admit Calabi–Yau compactifications. We give an alternative proof of this fact. As a consequence of this proof, we obtain a description of the fibers over infinity of the compactified toric Landau–Ginzburg models.

The problem of determining the kernel h(t), t ∈ [0, T], appearing in the system of integro-differential thermoviscoelasticity equations is considered. It is assumed that the coefficients of the equations depend only on one space variable. The inverse problem is replaced by the equivalent system of integral equations for unknown functions. The contraction mapping principle with weighted norms is applied to this system in the space of continuous functions. A global unique solvability theorem is proved and an estimate of the stability of the solution of the inverse problem is obtained.

We consider an initial boundary-value problem describing the unidirectional motion of a liquid in the Oberbeck–Boussinesq model in a plane channel with rigid immovable walls on which the temperature distribution is given (or the upper wall is heat-insulated). For this problem, we obtain a priori estimates, find an exact stationary solution, and determine conditions under which the solution converges to its stationary regime.

Balder’s well-known existence theorem (1983) for infinite-horizon optimal control problems is extended to the case in which the integral functional is understood as an improper integral. Simultaneously, the condition of strong uniform integrability (over all admissible controls and trajectories) of the positive part max{f₀, 0} of the utility function (integrand) f₀ is relaxed to the requirement that the integrals of f₀ over intervals [T, T′] be uniformly bounded above by a function ω(T, T′) such that ω(T, T′) → 0 as T, T′→∞. This requirement was proposed by A.V. Dmitruk and N.V. Kuz’kina (2005); however, the proof in the present paper does not follow their scheme, but is instead derived in a rather simple way from the auxiliary results of Balder himself. An illustrative example is also given.

The problem of finding new lower bounds for the degree of a branched covering of a manifold in terms of the cohomology rings of this manifold is considered. This problem is close to M. Gromov’s problem on the domination of manifolds, but it is more delicate. Any branched (finite-sheeted) covering of manifolds is a domination, but not vice versa (even up to homotopy). The theory and applications of the classical notion of the group transfer and of the notion of transfer for branched coverings are developed on the basis of the theory of n-homomorphisms of graded algebras.

The main result is a lemma imposing conditions on a relationship between the multiplicative cohomology structures of the total space and the base of n-sheeted branched coverings of manifolds and, more generally, of Smith–Dold n-fold branched coverings. As a corollary, it is shown that the least degree n of a branched covering of the N-torus T^N over the product of k 2-spheres and one (N − 2k)-sphere for N ≥ 4k + 2 satisfies the inequality n ≥ N − 2k, while the Berstein–Edmonds well-known 1978 estimate gives only n ≥ N/(k + 1).

The inequality

\(\ln {\kern 1pt} \ln \left( {r - \ln r} \right) + 1 < \mathop {\min }\limits_{0 < x \leqslant r - 1} \left( {\ln x + {x^{ - 1}}\ln \left( {r - x} \right)} \right) < \ln {\kern 1pt} \ln \left( {r - \ln \left( {r - {2^{ - 1}}\ln r} \right)} \right) + 1,\)

As is well known, the two-parameter Todd genus and the elliptic functions of level d define n-multiplicative Hirzebruch genera if d divides n + 1. Both cases are special cases of the Krichever genera defined by the Baker–Akhiezer function. In the present paper, the inverse problem is solved. Namely, it is proved that only these properties define n-multiplicative Hirzebruch genera among all Krichever genera for all n.

Let G be a finite group. A character χ of G is said to be real-imaginary if its values are real or purely imaginary. A conjugacy class C of a in G is real-imaginary if and only if χ(a) is real or purely imaginary for all irreducible characters χ of G. A finite group G is called real-imaginary if all of its irreducible characters are real-imaginary. In this paper, we describe real-imaginary conjugacy classes and irreducible characters and study some results related to the real-imaginary groups. Moreover, we investigate some connections between the structure of group G and both the set of all the real-imaginary irreducible characters of G and the set of all the real-imaginary conjugacy classes of G.

It is proved that if, for a subset A of a finite Abelian group G, under the action of a linear operator L: G³ → G², the image L(A, A, A) has cardinality less than (7/4)|A|², then there exists a subgroup H ⊆ G and an element x ∈ G for which A ⊆ H + x; further, |H| < (3/2)|A|.

In this paper, two known theorems on |N̄, p_n|_k summability methods of Fourier series have been generalized for |A, p_n|_k summability factors of Fourier series by using different matrix transformations. New results have been obtained dealing with some other summability methods.