Volume 513, Nº 1 (2023)

The article consists of observations regarding complete theories of countable signatures and their countable models. We provide a construction of a countable linearly ordered theory which has the same number of countable non-isomorphic models as the given countable, not necessarily linearly ordered, theory.

The paper presents two models of motion of a corporeal observer along a conical surface in \({{\mathbb{R}}^{3}}\), when the observed moving object has a set of high-speed hitting mini-objects.

The words “Programming is the second literacy” were coined more than 40 years ago [13], but never came to life. The paper develops and details that old slogan by proposing that the mainstream mathematics education in schools should merge with education in computer science/programming. Of course, this means a deep structural reform of school mathematics education. We are not talking about adapting the 20th century mathematics to the 21st century—s it outlined in [10, 19], we mean the 21st century mathematics education for the 21st century mathematics. To the best of our knowledge, this paper is perhaps the first known attempt to start a proper feasibility study for this reform. The scope of the paper does not allow us to touch the delicate socio-political (and financial) sides of the reform, we are looking only at general curricular and didactic aspects and possible directions of the reform. In particular, we indicate approaches to development of a Domain Specifiic Language (DSL) as a basis for all programming aspects of a new course.

The local dynamics of systems of two equations with delay is considered. The main assumption is that the delay parameter is large enough. Critical cases in the problem of the stability of the equilibrium state are highlighted and it is shown that they have infinite dimension. Methods of infinite-dimensional normalisation were used and further developed. The main result is the construction of special nonlinear boundary value problems which play the role of normal forms. Their nonlocal dynamics determines the behaviour of all solutions of the original system in а neighbourhood of the equilibrium state.

For a smooth projective curve \(\mathcal{C}\) defined over algebraic number field k, we investigate the question of finiteness of the set of generalized Jacobians \({{J}_{\mathfrak{m}}}\) of a curve \(\mathcal{C}\) associated with modules \(\mathfrak{m}\) defined over k such that a fixed divisor representing a class of finite order in the Jacobian J of the curve \(\mathcal{C}\) provides the torsion class in the generalized Jacobian \({{J}_{\mathfrak{m}}}\). Various results on the finiteness and infiniteness of the set of generalized Jacobians with the above property are obtained depending on the geometric conditions on the support of \(\mathfrak{m}\), as well as on the conditions on the field \(k\). These results were applied to the problem of the periodicity of a continuous fraction decomposition constructed in the field of formal power series \(k((1{\text{/}}x))\), for the special elements of the field of functions \(k(\tilde {\mathcal{C}})\) of the hyperelliptic curve \(\tilde {\mathcal{C}}:{{y}^{2}} = f(x)\).

We give a characterizations of Ramsey ultrafilters on ω in terms of functions \(f:{{\omega }^{n}} \to \omega \) and their ultrafilter extensions. To do this, we prove that for any partition \(\mathcal{P}\) of \({{[\omega ]}^{n}}\) there is a finite partition \(\mathcal{Q}\) of \({{[\omega ]}^{{2n}}}\) such that any set \(X \subseteq \omega \) that is homogeneous for \(\mathcal{Q}\) is a finite union of sets that are canonical for \(\mathcal{P}\).

An inverse spectral optimization problem is considered: for a given matrix potential \({{Q}_{0}}(x)\) it is required to find the matrix function \(\hat {Q}(x)\) closest to it, such that the k-th eigenvalue of the Sturm–Liouville matrix operator with potential \(\hat {Q}(x)\) matched the given value \(\lambda {\kern 1pt} *\). The main result of the paper is the proof of existence and uniqueness theorems. Explicit formulas for the optimal potential are established through solutions to systems of nonlinear differential equations of the second order, known in mathematical physics as systems of nonlinear Schrödinger equations

The article proposed a new model of the dynamics of growth of the World population, including discrete equations of the dynamics of percentage increases in integral volumes of inflow and outflow and a balance equation of population size. The principle of the dynamic balance of the demographic process and the condition of interval dynamic consistency based on this principle are formulated. A sample example of forecasting the growth of the World population in the period from 2011 to 2021 is given, demonstrating the possibility of building linear dynamic trends in the percentage increase in the integral volume of dead people, dynamically consistent with the corresponding intervals of statistics on the integral volumes of born children of earlier periods. Based on the proposed model, a forecast of the growth of the World population after 2021 was built, assuming that by 2050 the population will reach 9.466 billion, and in 2062 it will reach the maximum level of 9.561 billion, after which the World population will begin to decline and in 2100 will amount to 8.670 billion.