An analog of the Dougall formula and of the de Branges–Wilson integral

We derive a beta-integral over Z×R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathbb {Z}}\times {\mathbb {R}}$$\end{document}, which is a counterpart of the Dougall 5H5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_5H_5$$\end{document}-formula and of the de Branges–Wilson integral, our integral includes 10H10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{10}H_{10}$$\end{document}-summation. For a derivation we use a two-dimensional integral transform related to representations of the Lorentz group, this transform is a counterpart of the Olevskii index transform (a synonym: Jacobi transform).

i a −a π (a) (a ) sin πa . (1.1) Here a|a ∈ C Z × C. The C -function has poles at points a|a = −k| − l, where k, l ∈ N.

1.2)
We also can write the left-hand side as

Then the identity with the same right-hand side holds for
Re a α > 0, Re a α < 1. (1.4)

The de Branges-Wilson integral and the Dougall formula
Recall that the de Branges-Wilson integral is given by . (1.5) This formula was obtained by de Branges [4,5] in 1972, a proof was not published; the formula was rediscovered by Wilson [23], 1980; see also [1]. The Dougall 5 H 5 formula is (1.6) Setting b 4 + θ = 1, we get a series k≥0 of type 5 F 4 [. . . ; 1], this result was contained in a family of identities obtained by Dougall [8], 1906. It seems that the general bilateral formula was obtained by Bailey [3], 1936; see also [1].
Let us explain a similarity of such formulas. Denote by I (s) the integrand in (1.5) and extend it into a complex domain writing and we get the left-hand side of (1.6) with b α = 1 − a α . Also, we get the same right-hand sides.
In [21] (see formula (1.28)) there was derived a one-dimensional hybrid of (1.5) and (1.6) including both integration over the real axis and a summation over a lattice on the imaginary axis.
Our integral (1.2) can be obtained by formal replacing -factors in the integrand (1.5) by similar C -factors. The function C satisfies the reflection identity (1.7) Therefore and we come to the integrand in (1.2).

Further structure of the paper
In Sect. 2 we derive our integral (1.2). For a calculation we use a unitary integral transform J a,b defined in [18], see below Sect. 2.3. This transform is an analog of a classical integral transform known under the names generalized Mehler-Fock transform, Olevskii transform, Jacobi transform, see [16,17,19].
We write an appropriate family of functions H μ , and our integral (1.2) arises as the identity Section 3 contains a further discussion of Theorem 1.1.

Convergence of the integral
In particular, Remark Our expression is single valued. Indeed, But λ + λ ∈ Z, and therefore the result does not depend on a choice of a branch of ln λ.

The Gauss hypergeometric function of the complex field
For h|h ∈ C we denote Following [10] (see also [9], Sect. II.3.7) we define the beta function B C [·] and the Gauss hypergeometric function 2 F C 1 [·] of the complex field. Let a|a , b|b ∈ . Then The integral absolutely converges iff The right-hand side gives a meromorphic continuation of B C to the whole 2 For a|a , b|b , c|c ∈ we define the hypergeometric function (2. 2) The integral has an open domain of convergence on any connected component of the set of parameters 3 C Z 3 × C 3 , it admits a meromorphic continuation to the whole set 3 C , see [18], Sect. 3. The functions 2 F C 1 [·] admit explicit expressions in terms of sums of products of Gauss hypergeometric functions 2 F 1 . The standard properties of Gauss hypergeometric functions can be transformed to similar properties of functions 2 F C 1 , see [18], Sect. 3.
Below we need the following analog of the Gauss formula for 2 F 1 [a, b; c; 1]: see [18], Proposition 3.2, the last condition coincides with a condition of continuity

The index hypergeometric transform
Fix real a, b such that Consider the measure on C given by Consider the following function on Z × R: and consider the space L 2 even ( , κ a,b ) of even functions on with inner product Next, define the kernel on C × by In [18] there was obtained the following statement: The operator J a,b defined by is a unitary operator

Application of the Mellin transform
We define a Mellin transform M on C as the Fourier transform on the multiplicative group C × of C. Since C × (R/2π Z) × R, the Mellin transform is reduced to the usual Fourier transform and Fourier series. We have where ξ |ξ ∈ C . In the cases discussed below a function f on C\0 is differentiable except the point t = 1, where a singularity has a form Also in our cases the integral (2.7) absolutely converges for σ being in a certain strip A < [ξ |ξ ] < B. Therefore the Mellin transform is holomorphic in the strip. The inversion formula is given by the integration is taken over arbitrary line Re σ = γ , where A < γ < B. We understand the integral (which can be non-absolutely convergent) as The inversion formula holds at points of differentiability of f , also it holds at points of singularities of the form In this case we can repeat the standard proof of pointwise inversion formula for the one-dimensional Fourier transform and pointwise convergence of Fourier series, see, e. g., [15], Sect. VIII.1, VIII.3; for advanced multi-dimensional versions of the Dini condition, see, e.g., [24], Sect. 9.
Convolution f 1 * f 2 on C × is defined by As usual, we have this identity holds in intersection of strips of holomorphy M( f 1 ) and M( f 2 ). We also define a function So we have the following corollary of the convolution formula: where the integration contour is contained in the intersection of domains of holomorphy of M f 1 and M f 2 .

Then the Mellin transform sends a function
see [18], Theorem 3.9. This gives us a strip of convergence of the Mellin transform. We must evaluate the integral (2.14) We change an order of the integration and integrate in z: Integrating in t we again meet a B C -function and after simple cancellations and an application of the reflection formula (1.7) we come to (2.9). The successive integration is valid under conditions (2.10). We must justify the change of order of integrations. In fact, (2.14) is absolutely convergent as a double integral, i.e., It is a special case of integral (2.14), we integrate it successively in z and in t under the same condition as for successive integration in (2.14).

Lemma 2.4 Let λ|λ ∈ and
Proof We apply formula (2.8) assuming We evaluate Mellin transforms of f 1 , f 2 applying Lemma 2.3. In the integrand in the right-hand side of (2.8) we get a product of two factors. The first factor is (2.17) The second factor is holomorphic in the strip It a, b are sufficiently small, then strips (2.17) and (2.18) have a non-empty intersection and we can apply formula (2.8). Two factors C (a + b − ξ |a + b − ξ ) cancel and we get a factor independent of ξ and the integral The integrand up to a constant factor is a Mellin transform of a function 2 F C 1 [. . . ; z]. By the inversion formula, integral (2.16) converts to For sufficiently small a we can apply the Gauss identity (2.3). Thus we get (2.16) for sufficiently small a, b > 0. Keeping in mind (2.11)-(2.13), we can easily verify that the integral in the left-hand side of (2.16) converges for Re a > 0, Re b > 0, Re μ > 0, Re(a + μ) < 1, Re(b + μ) < 1.
Thus, under these conditions the left-hand side is holomorphic. The right-hand side also is holomorphic in this domain. Therefore, they coincide.

Proof of Theorem 1.1
Let a ∈ R. Consider a function H μ (z) on C given by (2.19) The statement (a) is trivial, (b) is reduced to B C -function.
The J a,b -image of H μ is done by Lemma 2.4. Since J a,b is unitary, we have This is Theorem 1.1, where the parameters a 1 , a 2 , a 3 , a 4 are a, b,

Barnes-Ismagilov integrals
Let p ≤ q. Let a α |a α , b α |b α ∈ . Following Ismagilov [12], we define integrals of the form By [12], Lemma 2, such integrals admit a representation of the form where γ j (·) are products of -factors and parameters of hypergeometric functions  [7]. Such results apparently must be regarded as analogs of expansions in Wilson polynomials.
Our theorem is an example of a hypergeometric identity for 5  with a α − a α ∈ Z instead of (1.3) with the corresponding transformation of the right-hand side. So we expect four additional integer parameters. Our calculation allows to introduce two additional discrete parameters, namely we can replace H μ by (1 − z) −a−μ|−a−μ . However, this is not sufficient to get a full generality. Apparently, the integral transform (2.6) admits an adding of discrete parameters. Some beta-integrals of type (1.2) with products of C -functions were obtained by Kels [13,14], and the integral (1.2) was obtained in another way by Derkachov, Manashov, and Valinevich in [6].

A difference problem
The de Branges-Wilson integral, the Dougall formula, and our integral (1.2) are representatives of beta integrals in the sense of Askey [2]. Quite often integrands w(x) in beta integrals are weight functions for systems of hypergeometric orthogonal polynomials. In particular, orthogonal polynomials corresponding to the de Branges-Wilson integral are the Wilson polynomials, see [1,23]. Recall that they are even eigenfunctions of the following difference operator: where i 2 = −1. If an integrand w(x) of a beta integral decreases as a power function, then only finite number of moments x n w(x) dx converge; however in this case a beta integral can be a weight for a finite system of hypergeometric orthogonal polynomials (this phenomenon was firstly observed by Romanovski in [22]), the system of orthogonal polynomials related to the Dougall 5 H 5 formula was obtained in [19]. On the other hand, such finite systems are discrete parts of spectra of explicitly solvable Sturm-Liouville problems (see, e.g., [11,20]). In the case of our integral (1.2) the integrand decreases as |k + is| 2 a j −8 , and we have no orthogonal polynomials. However a difference Sturm-Liouville problem can be formulated. We consider a space of meromorphic even functions (λ|λ ) on C , a weight on ⊂ C defined by the integrand (1.2), and the following commuting difference operators: See a simpler pair of difference operators of this kind in [18]. On the other hand, see a one-dimensional operator with continuous spectrum similar to L in Groenevelt [11].