Group of continuous transformations of real interval preserving tails of G₂-representation of numbers

In the paper, we consider a two-symbol system of encoding for real numbers with two bases having different signs g₀ < 1 and g₁ = g₀ − 1. Transformations (bijections of the set to itself) of interval [0, g₀] preserving tails of this representation of numbers are studied. We prove constructively th...

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Автори: Pratsiovytyi, M.V., Lysenko, I.M., Maslova, Yu.P.
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Назва видання:Algebra and Discrete Mathematics
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Цитувати:Group of continuous transformations of real interval preserving tails of G₂-representation of numbers / M.V. Pratsiovytyi, I.M. Lysenko, Yu.P. Maslova // Algebra and Discrete Mathematics. — 2020. — Vol. 29, № 1. — С. 99–108. — Бібліогр.: 10 назв. — англ.

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spelling irk-123456789-1885052023-03-04T01:27:15Z Group of continuous transformations of real interval preserving tails of G₂-representation of numbers Pratsiovytyi, M.V. Lysenko, I.M. Maslova, Yu.P. In the paper, we consider a two-symbol system of encoding for real numbers with two bases having different signs g₀ < 1 and g₁ = g₀ − 1. Transformations (bijections of the set to itself) of interval [0, g₀] preserving tails of this representation of numbers are studied. We prove constructively that the set of all continuous transformations from this class with respect to composition of functions forms an infinite non-abelian group such that increasing transformations form its proper subgroup. This group is a proper subgroup of the group of transformations preserving frequencies of digits of representations of numbers. 2020 Article Group of continuous transformations of real interval preserving tails of G₂-representation of numbers / M.V. Pratsiovytyi, I.M. Lysenko, Yu.P. Maslova // Algebra and Discrete Mathematics. — 2020. — Vol. 29, № 1. — С. 99–108. — Бібліогр.: 10 назв. — англ. 1726-3255 DOI:10.12958/adm1498 2010 MSC: 11H71, 26A46, 93B17 http://dspace.nbuv.gov.ua/handle/123456789/188505 en Algebra and Discrete Mathematics Інститут прикладної математики і механіки НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
description In the paper, we consider a two-symbol system of encoding for real numbers with two bases having different signs g₀ < 1 and g₁ = g₀ − 1. Transformations (bijections of the set to itself) of interval [0, g₀] preserving tails of this representation of numbers are studied. We prove constructively that the set of all continuous transformations from this class with respect to composition of functions forms an infinite non-abelian group such that increasing transformations form its proper subgroup. This group is a proper subgroup of the group of transformations preserving frequencies of digits of representations of numbers.
format Article
author Pratsiovytyi, M.V.
Lysenko, I.M.
Maslova, Yu.P.
spellingShingle Pratsiovytyi, M.V.
Lysenko, I.M.
Maslova, Yu.P.
Group of continuous transformations of real interval preserving tails of G₂-representation of numbers
Algebra and Discrete Mathematics
author_facet Pratsiovytyi, M.V.
Lysenko, I.M.
Maslova, Yu.P.
author_sort Pratsiovytyi, M.V.
title Group of continuous transformations of real interval preserving tails of G₂-representation of numbers
title_short Group of continuous transformations of real interval preserving tails of G₂-representation of numbers
title_full Group of continuous transformations of real interval preserving tails of G₂-representation of numbers
title_fullStr Group of continuous transformations of real interval preserving tails of G₂-representation of numbers
title_full_unstemmed Group of continuous transformations of real interval preserving tails of G₂-representation of numbers
title_sort group of continuous transformations of real interval preserving tails of g₂-representation of numbers
publisher Інститут прикладної математики і механіки НАН України
publishDate 2020
url http://dspace.nbuv.gov.ua/handle/123456789/188505
citation_txt Group of continuous transformations of real interval preserving tails of G₂-representation of numbers / M.V. Pratsiovytyi, I.M. Lysenko, Yu.P. Maslova // Algebra and Discrete Mathematics. — 2020. — Vol. 29, № 1. — С. 99–108. — Бібліогр.: 10 назв. — англ.
series Algebra and Discrete Mathematics
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fulltext “adm-n1” — 2020/5/14 — 19:35 — page 99 — #107 © Algebra and Discrete Mathematics RESEARCH ARTICLE Volume 29 (2020). Number 1, pp. 99–108 DOI:10.12958/adm1498 Group of continuous transformations of real interval preserving tails of G2-representation of numbers M. V. Pratsiovytyi, I. M. Lysenko, and Yu. P. Maslova Communicated by A. P. Petravchuk Dedicated to the 70th anniversary of Leonid Andriyovych Kurdachenko Abstract. In the paper, we consider a two-symbol system of encoding for real numbers with two bases having different signs g0 < 1 and g1 = g0 − 1. Transformations (bijections of the set to itself) of interval [0, g0] preserving tails of this representation of numbers are studied. We prove constructively that the set of all continuous transformations from this class with respect to compo- sition of functions forms an infinite non-abelian group such that increasing transformations form its proper subgroup. This group is a proper subgroup of the group of transformations preserving frequencies of digits of representations of numbers. Introduction Many two-symbol systems of encoding (representation) of fractional part of real numbers are known. They use the alphabet A = {0, 1} and are based on expansions of numbers in series, infinite products, continued fractions, etc. These systems identify a number as a sequence of zeros and 2010 MSC: 11H71, 26A46, 93B17. Key words and phrases: two-symbol system of encoding for real numbers with two bases having different signs (G2-representation), tail of representation of number, continuous transformation of interval, left and right shift operators, continuous transformation preserving tails of representations. https://doi.org/10.12958/adm1498 “adm-n1” — 2020/5/14 — 19:35 — page 100 — #108 100 Group of transformations preserving tails ones, i.e., as element of space L = A×A×. . . of sequences of zeros and ones. Every such a system has some advantages and restrictednesses as well as certain conveniences for solving some problems of number-theoretical, topological-metric and probabilistic kind. A system with two positive bases (q0 ∈ (0, 1), q1 = 1− q0) is among them. It generalizes classic binary system, has a self-similar geometry and zero redundancy. This is a system of Q2-representation of numbers based on representation of a number by the series x = α1q1−α1 + ∞∑ k=2 (αkq1−αk k−1∏ j=1 qαj ) ≡ ∆Q2 α1α2...αk... . For q0 = 0,5, it is a classic binary system. In expression ∏k j=1 qαj = q N0(x,k) 1 q N1(x,k) 1 , where N1(x, k) = α1 + α2 + . . .+αk and N0(x, k) = k−N1(x, k), one can see that this is a system with two bases. This system of encoding of numbers has various applications in metric and probabilistic theory of numbers, function theory and measure theory, fractal analysis and fractal geometry. For this system, the left shift operator is a discontinuous function, linear and increasing on cylinders of rank 1: ∆Q2 0 = [0, q0] and ∆Q2 1 = [q0, 1]. In the papers [7], [8], an analogue of Q2-representation of numbers, namely, system of encoding of numbers in interval [0, g0] with bases g0 ∈ (0, 1) and g1 = g0 − 1 having different signs is introduced. It is based on expansion of a number in the series x = α1g1−α1 + ∞∑ k=2 (αkg1−αk k−1∏ j=1 gαj ) ≡ ∆G2 α1α2...αk... . (1) This system also has a zero redundancy (any number has at most two rep- resentations, and set of numbers having two representations is countable). Generally speaking, series (1) has positive as well as negative terms. So G2-representation is not topologically equivalent to Q2-representation and it is not a simple reencoding of Q2-representation. At the same time metric theories of these representations are similar. A vivid peculiarity of G2-representation is the fact that left shift operator of G2-representation is a continuous function on a whole interval [0, g0]. This is a fundamental difference between this system and previously studied systems. Real numbers in interval [0, g0] having two G2-representations are called G2-binary numbers, and numbers having a unique representation “adm-n1” — 2020/5/14 — 19:35 — page 101 — #109 M. V. Pratsiovytyi, I . M. Lysenko, Yu. P. Maslova 101 are called G2-unary numbers. It is known [10] that the set B of all G2- binary numbers is countable and consists of numbers with the following G2-representation: ∆G2 c1...cm01(0) = ∆G2 c1...cm11(0). Algorithm for comparison of numbers in terms of their G2-representa- tions is the following. Theorem 1 ([10]). Numbers ∆G2 c1c2...cm1d1d2... = x1 6= x2 = ∆G2 c1c2...cm0d′ 1 d′ 2 ... are in the relation x1 > x2 if σm ≡ c1 + c2 + . . .+ cm = 2k, x1 6 x2 if σm ≡ c1 + c2 + . . .+ cm = 2k − 1. In this paper, we consider continuous transformations of interval [0, g0], i.e., bijections of interval to itself, preserving “tails of G2-represen- tation of numbers”, namely, we study group properties of the family of these transformations. Groups of transformations preserving tails of different representations of numbers were studied in the papers [2], [3], [4]. Specific properties of G2-representation generate a peculiar corresponding transformation group having a non-trivial subgroup of increasing functions unlike other systems. 1. Tails and tail sets Definition 1. We say that G2-representations of numbers x = ∆G2 α1...αn... and y = ∆G2 β1β2...βn... have the same tail if there exist positive integers k and m such that αk+j = βm+j (2) for any j ∈ N . We denote it symbolically by x ∼ y. Definition 2. Suppose that k and m are the smallest numbers satisfying condition (2). Then G2-representation and corresponding number z ≡ [x ∧ y] = ∆G2 αk+1αk+2... = ∆G2 βm+1βm+2... is called a common tail of representations of numbers x and y. It is evident that binary relation ∼ (“has the same tail”) is an equiva- lence relation, i.e., it is reflexive, symmetric and transitive. Thus it provides a partition of the set Z of all G2-representations of numbers in interval [0, g0] into the equivalence classes, and they form together a quotient set W = Z� ∼. Any element of the set W is called a tail set, this set is uniquely determined by an arbitrary its element. “adm-n1” — 2020/5/14 — 19:35 — page 102 — #110 102 Group of transformations preserving tails Theorem 2. Any tail set is a countable everywhere dense in interval [0, g0] set. The set W of all tail sets is a continuum set. Proof. Suppose Kx is a tail set containing the number x, K1 is a set of all numbers y such that [y ∧ x] = x, and Kn is a set of all numbers y such that [y ∧∆G2 αn+1(x)αn+2(x)... ] = ∆G2 αn+1(x)αn+2(x)... . It is evident that every set Kn (n ∈ N) is countable and Kx = ⋃ n Kn. Thus Kx is a countable set because it is a countable union of countable sets. An arbitrary cylinder ∆G2 c1...cn contains points of tail set Kx because points ∆G2 c1...cnα1(x)α2(x)... , ∆G2 c1...cnα2(x)α3(x)... , and ∆G2 c1...cnαk(x)αk+1(x)... be- long to this cylinder. This proves that the set Kx is everywhere dense in interval [0, g0]. The set W is a continuum set. Indeed, suppose that it is a countable set. Then we see that interval [0, g0] is a countable set because it is a countable union of countable sets. This contradicts the fact that interval is a continuum set. Note that all G2-binary numbers belong to the same tail set. 2. Left and right shift operators of G2-representation of numbers Theorem 3 ([8]). Left shift operator ω of G2-representation of numbers in interval [0, g0] defined by equality ω(∆G2 α1α2...αn... ) = ∆G2 α2α3...αn... in space of G2-representations, is analytically expressed in the form ω(x) = 1 gα1(x) x− δα1(x) gα1(x) , is a continuous well-defined function on [0, g0], is linear on every cylinder of rank 1, is increasing on ∆0 and decreasing on ∆1. Numbers 0 = ∆G2 (0) and ∆G2 (1) = g0 + g0g1 + g0g 2 1 + ... = g0 2 + g0 are invariant points of left shift operator ω. Remark 1. The last theorem shows an essential difference between G2- representation and other known two-symbol representations, in particular, Q∗ 2-representation, Q̃-representation [9] and A2-continued fractions [3]. “adm-n1” — 2020/5/14 — 19:35 — page 103 — #111 M. V. Pratsiovytyi, I . M. Lysenko, Yu. P. Maslova 103 Let n be a positive integer that is greater than 1. Put ωn(x) = ω(ωn−1(x)) = ∆G2 αn+1(x)αn+2(x)... . Since x = δα1(x) + n∑ k=2  δαk(x) k−1∏ j=1 gαj(x)  +   n∏ j=1 gαj(x)  ωn(x), we have ωn(x) = x Pn(x) − 1 Pn(x)  δα1(x) + n∑ k=2  δαk(x) k−1∏ j=1 gαj(x)     . Theorem 4 ([10]). Function ωn is well defined by equality ωn(x) = ωn(∆G2 α1(x)α2(x)...αn(x)... ) ≡ ∆G2 αn+1(x)αn+2(x)... , is analytically expressed in the form ωn(x) = 1 Pn x − Bn Pn , where Pn = ∏n j=1 gαj(x), Bn = δα1(x) + n∑ k=2 ( δαk(x) ∏k−1 j=1 gαj(x) ) , is continuous on interval [0, g0] and linear on every cylinder of rank n. Remark 2. In terms of dynamical systems, relation “has the same tail” can be defined in the following form: x = ∆G2 α1α2...αn ∼ y = ∆G2 β1β2...βn ⇔ Ox ∩Oy 6= ∅, where Ou = {u, ω1(u), ω2(u), . . .} is an orbit of point u under mapping ω, and [x ∧ y] = Ox ∩Oy. Definition 3. Function τi defined on [0, g0] by equality τi(x) = τi ( ∆G2 α1(x)α2(x)...αn(x)... ) = ∆G2 iα1(x)α2(x)...αn(x)... , where i ∈ {0, 1}, is called a right shift operator of G2-representation of numbers with parameter i (in the sequel, we just say “right shift operator”). Since τi ( ∆G2 c1c2...cm01(0) ) = τi ( ∆G2 c1c2...cm11(0) ) , we see that function τi is well defined. It is evident that the set of values of function τi is cylinder ∆G2 i . In particular, τ0(0) = τ ( ∆G2 (0) ) = 0, τ1(0) = τ1 ( ∆G2 (0) ) = ∆G2 1(0) = g0. “adm-n1” — 2020/5/14 — 19:35 — page 104 — #112 104 Group of transformations preserving tails Lemma 1. Function τi is continuous at any point of interval [0, g0] and analytically expressed in the form τi(x) = δi + gix. Indeed, since τi ( ∆G2 α1α2...αn... ) = ∆G2 iα1α2...αn... = δi + gi∆ G2 α1α2...αn... , we have τi(x) = δi + gix, i.e., τ0(x) = g0x, τ1(x) = g0 + g1x. So, it is evident that function τi is continuous on cylinders of rank 1. From equality τi ( ∆G2 c1c2...cm01(0) ) = τi ( ∆G2 c1c2...cm11(0) ) it follows that the function is continuous at G2-binary points that are endpoints of cylinders. Corollary 1. Function τ0 is increasing and function τ1 is decreasing. Moreover, τ0(g0) = τ1(g0). The following equalities are evident: ω(δi(x)) = x and τα1(x)(ω(x)) = x. Equation τi(x) = ω(x) has two solutions: x = ∆G2 (ji), where j ∈ {0, 1}. Equation τi(x) = ωm(x) has 2m solutions: x = ∆G2 (j1,j2,...,jmi), where jk ∈ {0, 1}, k = 1,m. Let (i1, i2, . . . , in) be a tuple of zeros and ones. Function τi1i2...in defined by equality τi1i2...in(x) = ∆G2 i1i2...inα1(x)α2(x)... is called a right shift operator with parameters (i1, i2, . . . , in). By induction, from equality τi1i2...in(x) = τi1(τi2...in(x)) it follows that operator τi1i2...in(x) is well defined. Operator τi1i2...in is analytically expressed in the form τi1i2...in(x) = δi1 + n∑ k=1  δik k−1∏ j=1 gij  +   n∏ j=1 gij  x, is a linear function, is increasing if Pn = ∏n j=1 gij > 0 (this is equivalent to i1 + i2 + . . . + in is even number) and decreasing if Pn < 0 (this is equivalent to i1 + i2 + . . .+ in is odd number). For example, consider n = 2 and corresponding functions τ00, τ01, τ10, τ11. Functions τ00 = g20x and τ11 = g20x + g20 are linear increasing, but functions τ01 = g0g1x+ g20 and τ10 = g0g1x+ g0 are linear decreasing. 3. Continuous functions and transformations of interval [0, g0] preserving tails of G2-representation of numbers We say that function y = f(x) preserves tails of G2-representation of numbers in interval [0, g0] (or is a tail function) if any number x ∈ [0, g0] and its image y = f(x) have the same tail. “adm-n1” — 2020/5/14 — 19:35 — page 105 — #113 M. V. Pratsiovytyi, I . M. Lysenko, Yu. P. Maslova 105 Left and right shift operators ωn, τi1i2...in for any positive integer n and for any tuple (i1, i2, . . . , in) of zeros and ones are simple examples of continuous functions preserving tails of G2-representation of numbers. Various “joinings” of these functions are the same. For example, function f(x) =    ω(x) if 0 6 x 6 x1, τ1(x) if x1 6 x 6 x2, ω(x) if x2 6 x 6 g0, where x1 and x2 are solutions of equation ω(x) = τ1(x), i.e., x1 = ∆G2 (01), x2 = ∆G2 (1), preserves tails of G2-representation. But not every continuous function defined on interval [0, g0] is its transformation, i.e., bijection of the interval to itself. It is clear that above mentioned functions are not transformations. It is clear that continuous transformations of interval [0, g0] can be only strictly monotonic (increasing and decreasing) functions such that their domain and set of values coincide with this interval. Lemma 2. Decreasing function f1(x) =    τ1(x) if x 6 x1 = ∆G2 (1) = g0 2− g0 , ω(x) if x > x1 = ∆G2 (1) = g0 2− g0 , is a continuous tail transformation of interval [0, g0]. Proof. Number x1 is a solution of equation τ1(x) = ω(x) being equivalent to system of equations 1 = α2(x) = α4(x) = . . . , α1(x) = α3(x) = α5(x) = . . .. So this equation has two solutions: x = ∆G2 (α11) , α1 ∈ {0, 1}. First solution x0 = ∆G2 (01) belongs to interval of decrease of function τ1 and to interval of increase of function ω, and second solution x1 belongs to interval of decrease of function τ1 and to interval of decrease of function ω. Thus f1 is a continuous and strictly decreasing function. Moreover, f1(0) = g0 and f1(g0) = 0. Hence f1 is a continuous transformation of interval [0, g0]. Example 1. Function f2(x) = { τ1(x) if 0 6 x 6 ∆G2 (101), ω2(x) if ∆G2 (101) 6 x 6 g0, is a continuous decreasing tail transformation. “adm-n1” — 2020/5/14 — 19:35 — page 106 — #114 106 Group of transformations preserving tails Indeed, τ1 is a continuous decreasing tail function and ∆G2 (101) is a so- lution of equation τ1(x) = ω2(x) belonging to the last interval of decrease of function ω2(x). Example 2. Decreasing function f3(x) =    τ10 . . . 0 ︸ ︷︷ ︸ k (x) if 0 6 x 6 xk ≡ ∆G2 i10 . . . 0 ︸ ︷︷ ︸ k , ω(x) if xk 6 x 6 g0, i ∈ A, is a continuous tail transformation of interval [0, g0]. Indeed, ω and τ10...0 are continuous tail functions, number xk is a so- lution of equation τ10 . . . 0 ︸ ︷︷ ︸ k (x) = ω(x) belonging to intervals of decrease of both functions. Hence f3 is a continuous decreasing tail function. Theorem 5. The set C of all continuous bijections of interval [0, g0] preserving tails of G2-representation of numbers with respect to composition (superposition) ◦ forms an infinite non-abelian group such that increasing functions form its non-trivial subgroup. Proof. It is known that the set of all bijections of interval forms a group such that an identity transformation is its neutral element and an inverse transformation is its symmetric element. It is evident that composition of tail transformations is a tail transformation. The same is true for inverse transformation. Thus, by the subgroup test, (C, ◦) is a group. From example 3 (where k is an arbitrary positive integer) it follows that this group is infinite. To prove that group (C, ◦) is non-abelian it is enough to provide two transformations in the set C that are not commute. To this end we consider function f1 and f2 from examples 1 and 2 and number x0 that is less than x1. Then we have f2(f1(∆ G2 01(0))) = f2(τ1(∆ G2 01(0))) = f2(∆ G2 101(0)) = τ1(∆ G2 101(0)) = ∆G2 1101(0) because of ∆G2 101(0) < ∆G2 (101); f1(f2(∆ G2 01(0))) = f1(τ1(∆ G2 01(0))) = f1(∆ G2 101(0)) = ω(∆G2 101(0)) = ∆G2 01(0), “adm-n1” — 2020/5/14 — 19:35 — page 107 — #115 M. V. Pratsiovytyi, I . M. Lysenko, Yu. P. Maslova 107 because of ∆G2 101(0) > ∆G2 (1). Hence f2(f1(x0)) 6= f1(f2(x0)). To prove that there exists a non-trivial subgroup of increasing functions it is enough to give an example of non-trivial increasing bijection f ∈ C. A such function is the following: f5(x) =    ω(x) if 0 6 x 6 ∆G2 (011), τ11(x) if ∆G2 (011) 6 x 6 ∆G2 (1), x if ∆G2 (1) 6 x 6 g0, because functions ω, τ11 and f(x) = x are increasing on given intervals, ∆G2 (011) is a solution of equation ω(x) = τ11(x), and ∆G2 (1) is a solution of equation τ11(x) = x. Remark 3. The group (C, ◦) is a proper subgroup of the group of transfor- mations of interval [0, g0] preserving frequencies of digits of representation. References [1] M. Iosifescu, C. Kraaikamp, Metric properties of Denjoy’s canonical continued fraction expansion, Tokyo J. Math., 31, no. 2, 2008, pp. 495–510. [2] T. M. Isaieva, M. V. Pratsiovytyi, Transformations of (0, 1] preserving tails ∆µ- representation of numbers, Algebra Discrete Math., 22, no. 1, 2016, pp. 102–115. [3] M. Pratsiovytyi, A. Chuikov, Continuous distributions whose functions preserve tails of an A-continued fraction representation of numbers, Random Oper. Stoch. Equ., 27, no. 3, 2019, pp. 199–206. [4] R. Yu. Osaulenko, Group of transformations of interval [0, 1] preserving frequencies of digits of Qs-representation of numbers, Trans. Inst. Math. Natl. Acad. Sci. Ukraine, 13, no. 3, 2016, pp. 191–204 (in Ukrainian). [5] M. V. Pratsiovytyi, Random variables with independent Q2-symbols, Asymptotic methods in investigation of stochastic models, Inst. Math. Acad. Sci. Ukrainian SSR, Kyiv, 1987, pp. 92–102 (in Russian). [6] M. V. Pratsiovytyi, Fractal properties of distributions of random variables whose Q2- signs form a homogeneous Markov chain, Asymptotic analysis of random evolutions, Inst. Math. Acad. Sci. Ukraine, Kyiv, 1994, pp. 249–254 (in Ukrainian). [7] M. V. Pratsiovytyi, Yu. P. Maslova, On one generalization of system of Rademacher and Walsh functions, Mathematical problems of mechanics and computational mathematics. Trans. Inst. Math. Natl. Acad. Sci. Ukraine, 13, no. 3, 2016, pp. 146– 157 (in Ukrainian). [8] M. V. Pratsiovytyi, I. M. Lysenko, Yu. P. Maslova, Geometry of numerical series: Series as a model of a real number in a new two-symbol system of encoding of numbers, Mathematical problems of mechanics and computational mathematics. Trans. Inst. Math. Natl. Acad. Sci. Ukraine, 15, no. 1, 2018, pp. 132–146 (in Ukrainian). “adm-n1” — 2020/5/14 — 19:35 — page 108 — #116 108 Group of transformations preserving tails [9] M. V. Pratsiovytyi, Fractal approach in investigation of singular probability distri- butions, Natl. Pedagog. Dragomanov Univ. Publ., Kyiv, 1998 (in Ukrainian). [10] M. V. Pratsiovytyi, I. M. Lysenko, Yu. P. Maslova, Numeral system with two bases having different signs and related special functions, To appear in Mathematical problems of mechanics and computational mathematics. Trans. Inst. Math. Natl. Acad. Sci. Ukraine, 16, 2019 (in Ukrainian). Contact information Mykola V. Pratsiovytyi, Iryna M. Lysenko, Yuliya P. Maslova Institute of Mathematics, National Academy of Sciences of Ukraine, Tereschenkivska str. 3, Institute of Physics and Mathematics, National Pedagogical Mykhailo Drahomanov University, 9 Pyrohova St., Kyiv, 01601, Ukraine E-Mail(s): prats4444@gmail.com, iryna.pratsiovyta@gmail.com, julia0609mas@gmail.com Received by the editors: 21.11.2019 and in final form 11.01.2020. M. V. Pratsiovytyi, I. M. Lysenko, Yu. P. Maslova