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Exact product forms for the simple cubic lattice Green function: I

G S Joyce and R T Delves

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The analytical properties of the lattice Green function \fl G(n,n,n;w)={1\over \pi^3}\int_0^{\pi}\!\!\!\int_0^{\pi}\!\!\!\int_0^{\pi}{{\cos{n}\theta{_1}\cos{n}\theta{_2}\cos{n}\theta{_3}}\over{w-\cos \theta_1-\cos \theta_2-\cos \theta_3}}\, {\rm d}\theta_1\, {\rm d}\theta_2\, {\rm d}\theta_3\nonumber\\[-7pt]
are investigated, where n is an integer and w is a complex variable. In particular, it is demonstrated that G(n, n, n; w) is a solution of a fourth-order linear differential equation of the Fuchsian type. From this differential equation it is found that G(n, n, n; w) can be evaluated in terms of a product of two Heun functions {Hj(n, v):j = 1, 2}, where v\equiv v(w) = {1\over{w^2}}\left(1+\sqrt{1-{1\over w^2}}\, \right)^{-1}\left(1+\sqrt{1-{9\over w^2}} \,\right)^{-1}.\nonumber\\[-6pt]
A detailed discussion of the properties of {Hj(n, v):j = 1, 2} is then given. The Heun function results are used to prove that the product form for G(n, n, n; w) can be expressed in terms of complete elliptic integrals of the first and second kinds. It is also shown that G(n, n, n; w) can be written in the hypergeometric form \fl wG(n,n,n;w)={{(3n)!}\over{(3^n n!)^3}}\left[{w\over 3}\left(1-\sqrt{1-{9\over w^2}} \,\right)\right]^{3n}{}_2F_1\left({1\over 3},{2\over 3};n+1;\eta_{+}\right)\\
\times\, {}_2F_1\left({1\over 3},{2\over 3};n+1;\eta_{-}\right)
where \fl \eta_{\pm} \equiv \eta_{\pm}(w) = {1\over{8w^2}}\left[4w^2+(9-4w^2)\sqrt{1-{9\over{w^2}}}\pm 27\sqrt{1-{1\over{w^2}}} \,\right].
This formula is valid for varying values of w in the neighbourhood of w = , provided that the argument function η+(w) does not take real values in the interval (1, + ). Finally, this 2F1 product form is used to determine the asymptotic behaviour of G(n, n, n; w) as n.


PACS

05.50.+q Lattice theory and statistics (Ising, Potts, etc.)

02.30.Hq Ordinary differential equations

MSC

82C20 Dynamic lattice systems (kinetic Ising, etc.) and systems on graphs

34B27 Green functions

Subjects

Mathematical physics

Statistical physics and nonlinear systems

Dates

Issue 11 (19 March 2004)

Received 6 November 2003

Published 2 March 2004



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