Method for Obtaining Coefﬁcients of Powers of Bivariate Generating Functions

: In this paper, we study methods for obtaining explicit formulas for the coefﬁcients of generating functions. To solve this problem, we consider the methods that are based on using the powers of generating functions. We propose to generalize the concept of compositae to the case of generating functions in two variables and deﬁne basic operations on such compositae: composition, addition, multiplication, reciprocation and compositional inversion. These operations allow obtaining explicit formulas for compositae and coefﬁcients of bivariate generating functions. In addition, we present several examples of applying the obtained results for getting explicit formulas for the coefﬁcients of bivariate generating functions. The introduced mathematical apparatus can be used for solving different problems that are related to the theory of generating functions.


Introduction
Generating functions are a widely used and powerful tool for solving problems in combinatorics, mathematical analysis, statistics, etc. For example, methods of the theory of generating functions are used for solving problems in combinatorics, since generating functions allow obtaining a compact representation of discrete structures and process them. For the first time, methods of generating functions were applied by de Moivre to solve recurrence equations. Next, Euler expanded the methods of generating functions for solving research problems related to partitions. In this case, a generating function through its coefficients shows the value for some special numbers that have combinatorial interpretations. Great contributions to the development of methods of generating functions and their application for solving mathematical problems in combinatorics were made by Riordan [1], Comtet [2], Flajolet and Sedgewick [3], Wilf [4], Stanley [5], Egorychev [6], Lando [7] and other scientists.
Moreover, generating functions are the main means of describing polynomials. Polynomials are one of the basic mathematical objects, and they are used in different areas of pure and applied mathematics. In this case, the coefficients of the generating function for a polynomial show the form of the polynomial for the fixed values of its parameters. Many studies present various properties of polynomials and their generating functions (e.g., those by Boas and Buck [8], Roman [9], Srivastava [10][11][12] and Simsek [13][14][15]).
The development of new methods for obtaining explicit formulas for the coefficients of generating functions is relevant research trend. The solution of this problem allows finding explicit formulas for special numbers and polynomials that are described by these generating functions. Different approaches for solving this problem can be found in papers by the following authors: Srivastava [16,17], Boyadzhiev [18], Cenkci [19] and other scientists.
Note that many research papers that are devoted to combinatorial problems and generating functions use the coefficients of the powers of generating functions. The coefficients of the powers of generating functions were first obtained by Euler when he obtained the coefficient function for (1 + x + x 2 + . . . + x n + . . .) k . The Binomial theorem, which is also the power of the generating function (x + y), was known even before de Moivre and Euler's works were published. In addition, the coefficients of the powers of generating functions play an important role in performing the composition of the generating functions.
The following concepts are related to the coefficients of the powers of generating functions: 1.
Potential polynomials introduced by Comtet [2]: The potential polynomial P (k) n is the kth power of an exponential generating function (k is a complex number): For the coefficients of potential polynomials, there is a relationship with the Bell polynomials, but operations on such polynomials are not defined.

2.
Riordan arrays introduced by Shapiro et al. [20]: A Riordan array is a pair of gener- It forms an infinite matrix D = (d n,k ) n,k≥0 where d n,k = [x n ]F(x)G(x) k . If we consider the associated subgroup of the Riordan group D = (1, G(x)), then we get However, there are no universal rules for obtaining explicit formulas for d n,k .

3.
Power matrices introduced by Knuth [21]: The power matrix of a given power series V(x) = V 1 x + V 2 x 2 + . . . is the infinite array of coefficients v n,k = n! k! [x n ]V(x) k . Thus, the kth power of V(x) can be presented in the form In addition, there are formulas for obtaining the coefficients w n,k for the composition of power series W(z) = U(V(z)) and for a power series of the form W(x) = αU(βx). However, the development of this mathematical apparatus is not presented.

4.
Compositae introduced by Kruchinin [22,23]: The composita F ∆ (n, k) of a generating function F(x) = ∑ n>0 f (n, k)x n is a coefficients function of its kth power: For two generating functions F(x) and G(x) and their compositae F ∆ (n, k) and G ∆ (n, k), we can find the composita A ∆ (n, k) of the generating function A(x) for the following cases: • addition of generating functions: A(x) = F(x) + G(x); • multiplication of generating functions: A(x) = F(x) · G(x); • composition of generating functions: A(x) = G(F(x)); • reciprocation of generating functions: A(x) · F(x) = 1; • compositional inversion of generating functions: F(A(x)) = x. In this way, it is possible to obtain an explicit formula for the composita for various types of generating functions.
It is also worth noting the results derived by M. Drmota [24] and aimed at obtaining an asymptotic expansion of the coefficients of the powers of generating functions.
However, all these concepts mainly consider generating functions in one variable, while there are many problems associated with generating functions in several variables (multivariate generating functions). Attempts to systematize the process of finding the coefficients of multivariate generating functions were made by Pemantle et al. [25], but they also investigated asymptotic methods for solving this problem. More on asymptotics derived from multivariate generating functions can be found in [26].
To solve this problem explicitly, we propose to generalize the concept of compositae to the case of generating functions in two variables (bivariate generating functions). The effectiveness of applying compositae is shown in obtaining explicit formulas for the coefficients of many ordinary generating functions that are related to special numbers and polynomials [27][28][29][30][31][32][33][34]. This research aims to improve and extend the original method.
The organization of this paper is as follows. Section 2 is devoted to a brief description of the proposed generalization of the concept of compositae. In Section 3, we introduce basic operations on compositae of bivariate generating functions: composition, addition, multiplication, reciprocation and compositional inversion. To confirm the effectiveness of using the proposed generalization of the concept of compositae, we present several examples of finding explicit formulas for coefficients of bivariate generating functions. The obtained results are shown in Section 4.

Composita of a Multivariate Generating Function
A multivariate generating function is the following formal power series: By ord(F), we denote the order of a formal power series F(x, y, . . . , z), which is defined as follows [35]: For two formal power series F(x, y, . . . , z) and G(x, y, . . . , z) with ord(F) > 0 and ord(G) > 0, the order of F(x, y, . . . , z) · G(x, y, . . . , z) is For a formal power series F(x, y, . . . , z) with ord(F) > 0, the order of F(x, y, . . . , z) k , k ∈ N, satisfies the inequality Next, we consider the coefficients of the kth power of a multivariate generating function In general, to calculate the coefficients f (n, m, . . . , l, k), we can use the method based on the following formula: Definition 1. The composita F ∆ (n, m, . . . , l, k) of a multivariate generating function is a coefficients function of the kth power of the generating function F(x, y, . . . , z): In addition, we set the following condition: F(x, y, . . . , z) 0 = 1. Hence, the composita F ∆ (n, m, . . . , l, k) for k = 0 is defined as follows: We can also write a recurrence for calculating compositae for k > 0 that is based on using the convolution operation for

Operations on Compositae of Bivariate Generating Functions
A bivariate generating function is the following formal power series: Using the concept of compositae, the kth power of the bivariate generating function F(x, y), with ord(F) ≥ 1, can be presented as follows: Next, we introduce basic operations on compositae of bivariate generating functions: composition, addition, multiplication, reciprocation and compositional inversion. These operations allow obtaining explicit formulas for compositae and coefficients of bivariate generating functions.

Composition of Bivariate Generating Functions
Theorem 1. Suppose that: Then, the coefficients g(n, m) of the composition of the bivariate generating functions Proof. Consider the given composition of bivariate generating functions Next, we represent the end part of this generating function as follows: To obtain an explicit formula for the coefficients c(n, m, k a , k b ), we apply the convolution operation and get Using Equation (1) and Inequality (2), we also have Then, taking into account the restriction for the indices of summation we can construct the following two summation schemes for obtaining the coefficients g(n, m): Combining the formula for g(n, m) with Equation (4), we obtain the desired result presented in Equation (3). Table 1 presents the obtained results for special cases of using the results of Theorem 1 for a bivariate generating function G(x, y) that is presented as the composition of bivariate and ordinary generating functions.
Next, we consider the process of finding coefficients for the kth power of a bivariate generating function that is obtained using the composition of bivariate generating functions. This result can be derived by generalizing the bivariate generating function H(x, y) from Theorem 1 to the case of its kth power. The obtained result is presented in Theorem 2. Table 1. Special cases of using the results of Theorem 1.

Composition Coefficient
Proof. The proof of Theorem 2 is similar to the proof of Theorem 1.
Corollary 1 can be applied for calculating the composita of a given bivariate generating function based on its decomposition into simpler functions. For example, it can be used for addition or multiplication of bivariate generating functions.

Addition of Bivariate Generating Functions
Theorem 3. Suppose that: Then, the composita G ∆ (n, m, k) of the addition of the bivariate generating functions G(x, y) = A(x, y) + B(x, y) = ∑ n≥0 ∑ m≥0 g(n, m)x n y m is equal to Proof. Consider a bivariate generating function H(x, y) = x + y and its kth power, that is based on the binomial theorem, where δ(i, j) is the Kronecker delta function: Applying Equation (6) Using the properties of the Kronecker delta function, we get k b = k − k a . Simplifying the formula for G ∆ (n, m, k), we obtain the desired result presented in Equation (7).

Multiplication of Bivariate Generating Functions
Theorem 4. Suppose that: Then, the composita G ∆ (n, m, k) of the addition of the bivariate generating functions G(x, y) = A(x, y) · B(x, y) = ∑ n≥0 ∑ m≥0 g(n, m)x n y m is equal to we obtain Using the properties of the Kronecker delta function, we get k a = k, k b = k. Simplifying the formula for G ∆ (n, m, k), we obtain the desired result presented in Equation (8).  Then, the coefficients g(n, m, k) are equal to

Reciprocation of Bivariate Generating Functions
Proof. Consider the kth power of the generating function G(x, y) as the following composition of generating functions: The coefficients h(n, k) can be calculating by h(n, k) = (−1) n n + k − 1 n .
Using the binomial theorem, the composita A ∆ (n, m, k) is equal to Applying Equation (5) for G(x, y), we obtain Then, we change the order of summation and get To remove the coefficient k a from the right binomial coefficient, we transform the binomial coefficient and obtain Next, we can simplify this formula by using the following identity (Identity (1.49) in [36]): Thus, we obtain the desired result presented in Equation (9).

Compositional Inversion of Bivariate Generating Functions
A compositional inverse F(x, y) of a bivariate generating function f (n, m)x n y m with respect to the variable x is a formal power series such that satisfies the condition F(F(x, y), y) = x.
Theorem 6. Suppose that: Then, the composita F ∆ (n, m, k) is equal to Proof. Using the Lagrange inversion theorem [37] for the functional equation In addition, we can represent this functional equation as follows: .
Applying Equation (9) for G(x, y), we get the coefficients of its kth power Combining Equation (11) with Equation (12), we obtain the desired result presented in Equation (10).

Application of Compositae for Obtaining Coefficients of Bivariate Generating Functions
Next, we present several examples of applying the obtained results for getting explicit formulas for coefficients of bivariate generating functions. The composita of the generating function A(x, y) = x + y is Applying Theorem 1, we can obtain the coefficients g(n, m) of the generating function G(x, y) If we have the generating function g(n, m) = h(n + m) n + m n = n + m n .
If we have the generating function then we get G(x, y) = H(x + y) = e x+y , g(n, m) = h(n + m) n + m n = 1 (n + m)! n + m n = 1 n!m! .
If we have the generating function of the Catalan numbers (the sequence A000108 in OEIS [38]) g(n, m) = h(n + m) n + m n = C n+m n + m n = 1 n + m + 1 2n + 2m n + m n + m n .

Example 2.
Let consider the generating function of the Eulerian numbers (the sequence A173018 in OEIS [38]) Consider this generating function E(x, y) as the following composition of generating functions: Using the identity for the Stirling numbers of the second kind we get the composita of the generating function B(x, y) Using the binomial theorem for we get the composita of the generating function C(x, y) Combining the obtained results and applying Theorem 1 for the composition B(x, C(x, y)), we obtain the composita of the generating function A(x, y) Using the properties of the Kronecker delta function, we get k a = k, k b = n − k.
Simplifying the formula for A ∆ (n, m, k), we obtain Applying Theorem 1 for the composition H (A(x, y)), we can obtain the following well-known explicit formula for the Eulerian numbers ( [39], Equation (6.40)): Example 3. Let consider the generating function of the Euler-Catalan numbers [40] (the sequence A316773 in OEIS [38]) is the generating function of the Catalan numbers.
Consider this generating function EC(x, y) as the following composition of generating functions: EC(x, y) = E(C(x), y), where E(x, y) is the generating function of the Eulerian numbers. The composita of the generating function C(x) is (cf. [22]) Applying Theorem 1 for the composition EC(C(x), y), we can obtain the following explicit formula for the Euler-Catalan numbers: Example 4. Let consider the generating function of the number triangle that forms the sequence A064189 in OEIS [38] G(x, y) = ∑ n≥0 ∑ m≥0 g(n, m)x n y m = 2 2x 2 is the generating function of the Motzkin numbers (the sequence A001006 in OEIS [38]).
Consider this generating function G(x, y) as the following composition of generating functions: G(x, y) = H(x, y) xy = B(M xy (x, y)) xy , The elements g(n, m) of this number triangle define the number of unlabeled binary trees with n internal nodes and exactly m distinguished external nodes.
Consider this generating function G(x, y) as the following composition of generating functions: A(x, y) = x + xy.
Using the binomial theorem for Finally, we get the coefficients g(n, m) of the generating function G(x, y) g(n, m) = h(n + 1, m) = C n n + 1 m = 1 n + 1 2n n n + 1 m .

Conclusions
This paper is devoted to the study of methods for obtaining explicit formulas for the coefficients of generating functions. To solve this problem, we consider the methods that are based on using the powers of generating functions. We propose to generalize the concept of compositae to the case of generating functions in two variables and define basic operations on such compositae: composition, addition, multiplication, reciprocation and compositional inversion. These operations allow obtaining explicit formulas for compositae and coefficients of bivariate generating functions. In addition, we present several examples of applying the obtained results for getting explicit formulas for coefficients of bivariate generating functions.
The introduced mathematical apparatus can be used for solving different problems that are related to the theory of generating functions. For example, it contributes to obtaining new explicit formulas for polynomials and special numbers. In addition, it can be applied for obtaining explicit formulas for the cardinality functions of combinatorial sets. This task is relevant in combinatorics, discrete mathematics and computer science when it is necessary to develop combinatorial generation algorithms [41].