In mathematics, a transformation of a sequence'sgenerating function provides a method of converting the generating function for one sequence into a generating function enumerating another. These transformations typically involve integral formulas applied to a sequence generating function (see integral transformations) or weighted sums over the higher-order derivatives of these functions (see derivative transformations).
In this article, we use the convention that the ordinary (exponential) generating function for a sequence is denoted by the uppercase function / for some fixed or formal when the context of this notation is clear. Additionally, we use the bracket notation for coefficient extraction from the Concrete Mathematics reference which is given by .
The main article gives examples of generating functions for many sequences. Other examples of generating function variants include Dirichlet generating functions (DGFs), Lambert series, and Newton series. In this article we focus on transformations of generating functions in mathematics and keep a running list of useful transformations and transformation formulas.
Extracting arithmetic progressions of a sequenceEdit
Series multisection provides formulas for generating functions enumerating the sequence given an ordinary generating function where , , and . In the first two cases where , we can expand these arithmetic progression generating functions directly in terms of :
More generally, suppose that and that denotes the primitive root of unity. Then we have the following formula, often known as the root of unity filter:
For integers , another useful formula providing somewhat reversed floored arithmetic progressions are generated by the identity
Powers of an OGF and composition with functionsEdit
The next formulas for powers, logarithms, and compositions of formal power series are expanded by these polynomials with variables in the coefficients of the original generating functions. The formula for the exponential of a generating function is given implicitly through the Bell polynomials by the EGF for these polynomials defined in the previous formula for some sequence of .
Reciprocals of an OGF (special case of the powers formula)Edit
The power series for the reciprocal of a generating function, , is expanded by
If we let denote the coefficients in the expansion of the reciprocal generating function, then we have the following recurrence relation:
Powers of an OGFEdit
Let be fixed, suppose that , and denote . Then we have a series expansion for given by
and the coefficients satisfy a recurrence relation of the form
If we let and define , then we have a power series expansion for the composite generating function given by
where the coefficients, , in the previous expansion satisfy the recurrence relation given by
and a corresponding formula expanded by the Bell polynomials in the form of the power series coefficients of the following generating function:
Faà di Bruno's formulaEdit
Let denote the EGF of the sequence, , and suppose that is the EGF of the sequence, . Faà di Bruno's formula implies that the sequence, , generated by the composition, can be expressed in terms of the exponential Bell polynomials as follows:
OGF ⟷ EGF conversion formulasEdit
We have the following integral formulas for which can be applied termwise with respect to when is taken to be any formal power series variable:
Notice that the first and last of these integral formulas are used to convert between the EGF to the OGF of a sequence, and from the OGF to the EGF of a sequence whenever these integrals are convergent.
The first integral formula corresponds to the Laplace transform (or sometimes the formal Laplace–Borel transformation) of generating functions, denoted by , defined in. Other integral representations for the gamma function in the second of the previous formulas can of course also be used to construct similar integral transformations. One particular formula results in the case of the double factorial function example given immediately below in this section. The last integral formula is compared to Hankel's loop integral for the reciprocal gamma function applied termwise to the power series for .
Example: A double factorial integral for the EGF of the Stirling numbers of the second kindEdit
where an integral for the double factorial function, or rational gamma function, is given by
for natural numbers . This integral representation of then implies that for fixed non-zero and any integral powers , we have the formula
Thus for any prescribed integer , we can use the previous integral representation together with the formula for extracting arithmetic progressions from a sequence OGF given above, to formulate the next integral representation for the so-termed modifiedStirling number EGF as
which is convergent provided suitable conditions on the parameter .
Example: An EGF formula for the higher-order derivatives of the geometric seriesEdit
For fixed non-zero defined such that , let the geometric series over the non-negative integral powers of be denoted by . The corresponding higher-order derivatives of the geometric series with respect to are denoted by the sequence of functions
for non-negative integers . These derivatives of the ordinary geometric series can be shown, for example by induction, to satisfy an explicit closed-form formula given by
for any whenever . As an example of the third OGF EGF conversion formula cited above, we can compute the following corresponding exponential forms of the generating functions :
Fractional integrals and derivativesEdit
Fractional integrals and fractional derivatives (see the main article) form another generalized class of integration and differentiation operations that can be applied to the OGF of a sequence to form the corresponding OGF of a transformed sequence. For we define the fractional integral operator (of order ) by the integral transformation
which corresponds to the (formal) power series given by
For fixed defined such that , we have that the operators .
Moreover, for fixed and integers satisfying we can define the notion of the fractional derivative satisfying the properties that
where we have the semigroup property that only when none of is integer-valued.
Polylogarithm series transformationsEdit
For fixed , we have that (compare to the special case of the integral formula for the Nielsen generalized polylogarithm function defined in) 
Notice that if we set , the integral with respect to the generating function, , in the last equation when corresponds to the Dirichlet generating function, or DGF, , of the sequence of provided that the integral converges. This class of polylogarithm-related integral transformations is related to the derivative-based zeta series transformations defined in the next sections.
Square series generating function transformationsEdit
For fixed non-zero such that and , we have the following integral representations for the so-termed square series generating function associated with the sequence , which can be integrated termwise with respect to :
This result, which is proved in the reference, follows from a variant of the double factorial function transformation integral for the Stirling numbers of the second kind given as an example above. In particular, since
we can use a variant of the positive-order derivative-based OGF transformations defined in the next sections involving the Stirling numbers of the second kind to obtain an integral formula for the generating function of the sequence, , and then perform a sum over the derivatives of the formal OGF, to obtain the result in the previous equation where the arithmetic progression generating function at hand is denoted by
for each fixed .
Hadamard products and diagonal generating functionsEdit
We have an integral representation for the Hadamard product of two generating functions, and , stated in the following form:
More information about Hadamard products as diagonal generating functions of multivariate sequences and/or generating functions and the classes of generating functions these diagonal OGFs belong to is found in Stanley's book. The reference also provides nested coefficient extraction formulas of the form
which are particularly useful in the cases where the component sequence generating functions, , can be expanded in a Laurent series, or fractional series, in , such as in the special case where all of the component generating functions are rational, which leads to an algebraic form of the corresponding diagonal generating function.
Example: Hadamard products of rational generating functionsEdit
Ordinary generating functions for generalized factorial functions formed as special cases of the generalized rising factorial product functions, or Pochhammer k-symbol, defined by
where is fixed, , and denotes the Pochhammer symbol are generated (at least formally) by the Jacobi-type J-fractions (or special forms of continued fractions) established in the reference. If we let denote the convergent to these infinite continued fractions where the component convergent functions are defined for all integers by
where denotes an associated Laguerre polynomial, then we have that the convergent function, , exactly enumerates the product sequences, , for all . For each , the convergent function is expanded as a finite sum involving only paired reciprocals of the Laguerre polynomials in the form of
Moreover, since the single factorial function is given by both and , we can generate the single factorial function terms using the approximate rational convergent generating functions up to order . This observation suggests an approach to approximating the exact (formal) Laplace–Borel transform usually given in terms of the integral representation from the previous section by a Hadamard product, or diagonal-coefficient, generating function. In particular, given any OGF we can form the approximate Laplace transform, which is -order accurate, by the diagonal coefficient extraction formula stated above given by
Examples of sequences enumerated through these diagonal coefficient generating functions arising from the sequence factorial function multiplier provided by the rational convergent functions include
We can also expand the negative-order zeta series transformations by a similar procedure to the above expansions given in terms of the -order derivatives of some and an infinite, non-triangular set of generalized Stirling numbers in reverse, or generalized Stirling numbers of the second kind defined within this context.
In particular, for integers , define these generalized classes of Stirling numbers of the second kind by the formula
Then for and some prescribed OGF, , i.e., so that the higher-order derivatives of exist for all , we have that
A table of the first few zeta series transformation coefficients, , appears below. These weighted-harmonic-number expansions are almost identical to the known formulas for the Stirling numbers of the first kind up to the leading sign on the weighted harmonic number terms in the expansions.
Examples of the negative-order zeta series transformationsEdit
Additional series representations for the r-order harmonic number exponential generating functions for integers are formed as special cases of these negative-order derivative-based series transformation results. For example, the second-order harmonic numbers have a corresponding exponential generating function expanded by the series
Generalized negative-order zeta series transformationsEdit
A further generalization of the negative-order series transformations defined above is related to more Hurwitz-zeta-like, or Lerch-transcendent-like, generating functions. Specifically, if we define the even more general parametrized Stirling numbers of the second kind by
for non-zero such that , and some fixed , we have that
Moreover, for any integers , we have the partial series approximations to the full infinite series in the previous equation given by
Examples of the generalized negative-order zeta series transformationsEdit
Series for special constants and zeta-related functions resulting from these generalized derivative-based series transformations typically involve the generalized r-order harmonic numbers defined by
for integers . A pair of particular series expansions for the following constants when is fixed follow from special cases of BBP-type identities as
for and where the golden ratio (and its reciprocal) are respectively defined by .
Inversion relations and generating function identitiesEdit
An inversion relation is a pair of equations of the form
which is equivalent to the orthogonality relation
Given two sequences, and , related by an inverse relation of the previous form, we sometimes seek to relate the OGFs and EGFs of the pair of sequences by functional equations implied by the inversion relation. This goal in some respects mirrors the more number theoretic (Lambert series) generating function relation guaranteed by the Möbius inversion formula, which provides that whenever
the generating functions for the sequences, and , are related by the Möbius transform given by
Similarly, the Euler transform of generating functions for two sequences, and , satisfying the relation
is given in the form of
where the corresponding inversion formulas between the two sequences is given in the reference.
The remainder of the results and examples given in this section sketch some of the more well-known generating function transformations provided by sequences related by inversion formulas (the binomial transform and the Stirling transform), and provides several tables of known inversion relations of various types cited in Riordan's Combinatorial Identities book. In many cases, we omit the corresponding functional equations implied by the inversion relationships between two sequences (this part of the article needs more work).
The binomial transformEdit
The first inversion relation provided below implicit to the binomial transform is perhaps the simplest of all inversion relations we will consider in this section. For any two sequences, and , related by the inversion formulas
we have functional equations between the OGFs and EGFs of these sequences provided by the binomial transform in the forms of
The Stirling transformEdit
For any pair of sequences, and , related by the Stirling number inversion formula
these inversion relations between the two sequences translate into functional equations between the sequence EGFs given by the Stirling transform as
Tables of inversion pairs from Riordan's bookEdit
These tables appear in chapters 2 and 3 in Riordan's book providing an introduction to inverse relations with many examples, though which does not stress functional equations between the generating functions of sequences related by these inversion relations. The interested reader is encouraged to pick up a copy of the original book for more details.
Several forms of the simplest inverse relationsEdit
The falling -binomial transform (refer to Spivey's article in )
The rising -binomial transform (refer to Spivey's article in )
Gould classes of inverse relationsEdit
The terms, and , in the inversion formulas of the form
forming several special cases of Gould classes of inverse relations are given in the next table.
For classes 1 and 2, the range on the sum satisfies , and for classes 3 and 4 the bounds on the summation are given by . These terms are also somewhat simplified from their original forms in the table by the identities
The simpler Chebyshev inverse relationsEdit
The so-termed simpler cases of the Chebyshev classes of inverse relations in the subsection below are given in the next table.
Inverse Formula for
The formulas in the table are simplified somewhat by the following identities:
Additionally the inversion relations given in the table also hold when in any given relation.
Chebyshev classes of inverse relationsEdit
The terms, and , in the inversion formulas of the form
for non-zero integers
forming several special cases of Chebyshev classes of inverse relations are given in the next table.
Additionally, these inversion relations also hold when for some or when the sign factor of is shifted from the terms to the terms . The formulas given in the previous table are simplified somewhat by the identities
The simpler Legendre inverse relationsEdit
Inverse Formula for
Legendre–Chebyshev classes of inverse relationsEdit
The Legendre–Chebyshev classes of inverse relations correspond to inversion relations of the form
where the terms, and , implicitly depend on some fixed non-zero . In general, given a class of Chebyshev inverse pairs of the form
if a prime, the substitution of , , and (possibly replacing ) leads to a Legendre–Chebyshev pair of the form
Similarly, if the positive integer is composite, we can derive inversion pairs of the form
The next table summarizes several generalized classes of Legendre–Chebyshev inverse relations for some non-zero integer .
Abel inverse relationsEdit
Abel inverse relations correspond to Abel inverse pairs of the form
where the terms, and , may implicitly vary with some indeterminate summation parameter .
These relations also still hold if the binomial coefficient substitution of is performed for some non-negative integer .
The next table summarizes several notable forms of these Abel inverse relations.
Generating Function Identity
Inverse relations derived from ordinary generating functionsEdit
If we let the convolved Fibonacci numbers, , be defined by