Abstract
By examining Fourier–Legendre series pairs and , fifteen classes of symmetric and asymmetric bivariate series will systematically be investigated. A compendium of 360 remarkable summation formulae will be established. They may serve as a complementary work to the paper by Chu and Campbell (The Ramanujan Journal (2023)).
Keywords:
hypergeometric series; Legendre polynomial; Fourier–Legendre series; product integration; symmetric double series; gamma function; central binomial coefficient; multinomial coefficient MSC:
33C45; 42C10; 11B65; 33C20; 65B10
1. Introduction and Outline
For and an indeterminate x, the shifted factorials are defined by
In general, for , we can express it as the -function ratio
According to Bailey [1], the generalized hypergeometric series reads as
where for the sake of brevity, the factorial quotient is abbreviated to
The Fourier–Legendre series plays important rules in mathematics, theoretical physics and applied sciences (see for example [2,3]). By integrating Fourier–Legendre series, Campbell [4] recently discovered the following elegant identity:
Subsequently, this method was systematically developed by Chu and Campbell [5] to evaluate, in closed form, numerous double series containing binomial and multinomial coefficients. Denoting by G the usual Catalan constant (cf. [6,7]), the authors confine themselves to reproduce the following remarkable ones (where the series in the middle is symmetric with respect to i and j):
Observe that the common feature among these double series is that they are almost tensor products of two independent sums in i and j, weakly tied by linear functions of i and j in denominators. We shall investigate, in this paper, another large class of double series representations for , consisting of two independent single sums in i and j, strongly tied by the binomial coefficient in denominators. Two typical examples are as follows:
They are two representatives of a totally new class of numerous double series identities (counting 360 exactly). In addition, it is worth highlighting that the former series possesses a perfect symmetry with respect to summation indices i and j, while the latter one is quasi symmetric. Numerous identities in the next section (see Section 2.1, Section 2.6, Section 2.10 and Section 2.13) have similar properties.
Suppose that and are two analytic functions in with power series expressions:
Then, it is not hard to check that and are also analytic in . Hence, we can expand these two composite functions in terms of Legendre polynomials (cf. [8,9] and Rainville [10], Chapter 10):
Recently, Campbell and the second author discovered the following remarkable formula [5] (Theorem 11). Letting the sequences and be as in (1), (2) and (3) under the given conditions of and , we have “the universal identity” of infinite series:
By combining this theorem with the forty Legendre–Fourier series shown in [5] (Corollaries 6–10), numerous double infinite series were evaluated in closed form in the just-cited paper.
Now, taking into account Equation (3), there is a companion Fourier–Legendre series
We can similarly examine the integral
On the other hand, consider the integral of the corresponding Fourier–Legendre series
where stands for the logical function with and . Therefore, we have established another general double series identity which serves as a counterpart of (4):
As the reader will observe in the sequel, this “universal formula” serves as a framework for a very large class of new double series, as illustrated by the preceding representatives. In particular, when , the above series becomes symmetric in i and j.
Analogous to what has been done in [5], by appropriately examining the Fourier–Legendre series pairs and (recorded in the Appendix A), numerous double series formulae will be established in Section 2 by employing (5). Finally, this paper will conclude with Section 3, where we shall present exceptional double series whose values involve the golden ratio.
2. Fifteen Classes of Double Series
By combining appropriately the “A, B, C, D and E” series displayed in Appendix A, we are going to derive 360 double infinite series identities in accordance with relation (5). They will be divided into fifteen classes according to the Fourier–Legendre series pairs.
2.1. Combined Products from Series in Class A
2.2. Crossing Products from Series in Classes A and B
2.3. Crossing Products from Series in Classes A and C
2.4. Crossing Products from Series in Classes A and D
2.5. Crossing Products from Series in Classes A and E
2.6. Combined Products from Series in Class B
2.7. Crossing Products from Series in Classes B and C
2.8. Crossing Products from Series in Classes B and D
2.9. Crossing Products from Series in Classes B and E
2.10. Combined Products from Series in Class C
2.11. Crossing Products from Series in Classes C and D
2.12. Crossing Products from Series in Classes C and E
2.13. Combined Products from Series in Class D
2.14. Crossing Products from Series in Classes D and E
2.15. Combined Products from Series in Class E
3. Concluding Comments
Besides squares of central binomial coefficients as appearing in the double series recorded in the last section, it is possible to examine other binomial expressions, as long as their generating functions can explicitly be expressed in terms of Legendre polynomials. The reader is encouraged to make further exploration.
As an example, we are going to examine some asymmetric series with their values involving golden ratio. For and , consider the binomial series
This function also admits the expansion in terms of Legendre polynomials
By multiplying the above series with (A1) displayed in Appendix A and then applying equality (4), we find two interesting double series. The first one reads as
Here and forth, stands for the dilogarithm function (cf. [11]) explicitly given by
When and , we obtain two infinite series identities:
where for the last one we have invoked two known values (cf. [11])
Alternatively, we have another double series
When , it reduces to the identity below
Author Contributions
Writing, editing and computations, M.B.; Original draft, review and supervision, W.C. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Data Availability Statement
Data are contained within the article.
Acknowledgments
The authors express their sincere gratitude to reviewers for the careful reading, critical comments, and valuable suggestions, in particular, accurately correcting several computation errors, that were not only precious to the authors, but also significant contributions to improving the manuscript during the revision.
Conflicts of Interest
The authors declare no conflicts of interest.
Appendix A. Forty Legendre–Fourier Series
In order to facilitate the reader’s understanding, we reproduce the five classes of forty Legendre–Fourier series shown in [5] (Corollaries 6–10), where all the series are convergent for .
- Legendre–Fourier Series: Class A (see Corollary 6 in [5])
- Legendre–Fourier Series: Class B (see Corollary 7 in [5])
- Legendre–Fourier Series: Class C (see Corollary 8 in [5])
- Legendre–Fourier Series: Class D (see Corollary 9 in [5])
- Legendre–Fourier Series: Class E (see Corollary 10 in [5])
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