Rational approximations to linear forms of exponentials and binomials

Let ξ \xi be a real irrational number. We are interested in sequences of linear forms in 1 and ξ \xi , with integer coefficients, which tend to 0. Does such a sequence exist such that the linear forms are small (with given rate of decrease) and the coefficients have some given rate of growth? If these rates are essentially geometric, a necessary condition for such a sequence to exist is that the linear forms are not too small, a condition which can be expressed precisely using the irrationality exponent of ξ \xi . We prove that this condition is actually sufficient, even for arbitrary rates of growth and decrease. We also make some remarks and ask some questions about multivariate generalizations connected to Fischler-Zudilin’s new proof of Nesterenko’s linear independence criterion.

Let $\mathcal{G}(z):=\sum_{n\geqslant0} z^{2^{n}}(1-z^{2^{n}})^{-1}$ denote the generating function of the ruler function, and $\mathcal {F}(z):=\sum_{n\geqslant} z^{2^{n}}(1+z^{2^{n}})^{-1}$ ; note that the special value $\mathcal{F}(1/2)$ is the sum of the reciprocals of the Fermat numbers $F_{n}:=2^{2^{n}}+1$ . The functions $\mathcal{F}(z)$ and $\mathcal{G}(z)$ as well as their special values have been studied by Mahler, Golomb, Schwarz, and Duverney; it is known that the numbers $\mathcal {F}(\alpha)$ and $\mathcal{G}(\alpha)$ are transcendental for all algebraic numbers α which satisfy 0<α<1. For a sequence u, denote the Hankel matrix $H_{n}^{p}(\mathbf {u}):=(u({p+i+j-2}))_{1\leqslant i,j\leqslant n}$ . Let α be a real number. The irrationality exponent μ(α) is defined as the supremum of the set of real numbers μ such that the inequality |α−p/q|<q −μ has infinitely many solutions (p,q)∈ℤ×ℕ. In this paper, we first prove that the determinants of $H_{n}^{1}(\mathbf {g})$ and $H_{n}^{1}(\mathbf{f})$ are nonzero for every n⩾1. We then use this result to prove that for b⩾2 the irrationality exponents $\mu(\mathcal{F}(1/b))$ and $\mu(\mathcal{G}(1/b))$ are equal to 2; in particular, the irrationality exponent of the sum of the reciprocals of the Fermat numbers is 2.

We shall obtain the irrationality exponent 2 for some values of two special Mahler functions. This gives a new proof for the recent result of Bugeaud on Thue–Morse–Mahler numbers.

In this paper we generalize Nesterenko’s criterion to the case where the small linear forms have an oscillating behaviour (for instance given by the saddle point method). This criterion provides both a lower bound for the dimension of the vector space spanned over the rationals by a family of real numbers and a measure of simultaneous approximation to these numbers (namely, an upper bound for the irrationality exponent if 1 and only one other number are involved). As an application, we prove an explicit measure of simultaneous approximation to ζ(5), ζ(7), ζ(9), and ζ(11), using Zudilin’s proof that at least one of these numbers is irrational.

Consider the classical problem of rational simultaneous approximation to a point in \({\mathbb {R}}^{n}\). The optimal lower bound on the gap between the induced ordinary and uniform approximation exponents has been established by Marnat and Moshchevitin in 2018. Recently Nguyen, Poels and Roy provided information on the best approximating rational vectors to the points where the gap is close to this minimal value. Combining the latter result with parametric geometry of numbers, we effectively bound the dual linear form exponents in the described situation. As an application, we slightly improve the upper bound for the classical exponent of uniform Diophantine approximation \({\widehat{\lambda }}_{n}(\xi )\), for even \(n\ge 4\). Unfortunately our improvements are small, for \(n=4\) only in the fifth decimal digit. However, the underlying method in principle can be improved with more effort to provide better bounds. We indeed establish reasonably stronger results for numbers which almost satisfy equality in the estimate by Marnat and Moshchevitin. We conclude with consequences on the classical problem of approximation to real numbers by algebraic numbers/integers of uniformly bounded degree.

При изучении арифметических свойств значений обобщенных гипергеометрических функций часто применяют известный в теории трансцендентных чисел метод Зигеля. Наиболее общие результаты в данной области были получены именно этим методом. Однако возможности метода Зигеля в случае гипергеометрических функций с иррациональнымипараметрами ограничены. Это связано с тем, что такие гипергеометрические функции не являются E-функциями, и по этой причине построить линейную приближающую форму свысоким порядком нуля с помощью принципа Дирихле здесь не удается. При рассмотрении задач, связанных с исследованием арифметической природы значений гипергеометрических функций с иррациональными параметрами, в некоторых случаях можно применить метод, основанный на эффективном построении линейной приближающей формы, но возможности этого метода также ограничены из-за того, что слишком общие эффективные конструкции отсутствуют. Трудности имеются также и в тех случаях, когда такие конструкции известны. Особенности этих конструкций таковы, что часто не удается реализовать арифметическую часть метода.Поэтому представляют интерес ситуации, когда можно провести требуемое исследование, опираясь на особые свойства конкретных гипергеометрических функций. Иногдаудается так подобрать параметры исследуемых функций, что можно преодолеть те трудности, которые возникают в общем случае. В настоящей работе рассматривается гипергеометрическая функция специального вида и ее производные. С помощью эффективной конструкции удалось не только доказать линейную независимость значений этих функций над некоторым мнимым квадратичным полем, но и получить соответствующий количественный результат в виде оценки модуля линейной формы от указанных значений.

The role of fractional calculus in circuit systems has received increased attention in recent years. In order to evaluate the effectiveness of time-domain calculation methods in the analysis of fractional-order piecewise smooth circuit systems, an experimental prototype is developed, and the effects of three typical calculation methods in different test scenarios are compared and studied in this paper. It is proved that Oustaloup’s rational approximation method usually overestimates the peak-to-peak current and brings in the pulse–voltage phenomenon in piecewise smooth test scenarios, while the results of the two iterative recurrence-form numerical methods are in good agreement with the experimental results. The study results are dedicated to provide a reference for efficiently deploying calculation methods in fractional-order piecewise smooth circuit systems. Some quantitative analysis results are concluded in this paper.

A result of Chudnovsky concerning rational approximation to certain algebraic numbers is reworked to provide a quantitative result in which all constants are explicitly given. More particularly, Padé approximants to the function ( 1 − x ) 1 / 3 {(1 - x)^{1/3}} are employed to show, for certain integers a and b , that \[ | ( a / b ) 1 / 3 − p / q | &gt; c q − κ when q &gt; 0. \left | {{{(a/b)}^{1/3}} - p/q} \right | &gt; c{q^{ - \kappa }}\quad {\text {when}}\,q &gt; 0. \] Here, c and k are given as functions of a and b only.