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Integrals of the Ising class

D H Bailey1, J M Borwein2 and R E Crandall3

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From an experimental-mathematical perspective we analyse 'Ising-class' integrals. These are structurally related n-dimensional integrals we call Cn, Dn, En, where Dn is a magnetic susceptibility integral central to the Ising theory of solid-state physics. We first analyse

C_n := \frac4{n!} \int_{0}^{\infty} \cdots \int_{0}^{\infty}
\frac{1} {\big(\!\sum_{j=1}^{n} (u_j + 1/u_j) \big)^2}
\frac{{\rm d}u_1}{u_1} \cdots \frac{{\rm d}u_n}{u_n}.

We had conjectured—on the basis of extreme-precision numerical quadrature—that Cn has a finite large-n limit, namely C = 2 e−2γ, with γ being the Euler constant. On such a numerological clue we are able to prove the conjecture. We then show that integrals Dn and En both decay exponentially with n, in a certain rigorous sense. While Cn, Dn remain unresolved for n ≥ 5, we were able to conjecture a closed form for E5. Our experimental results involved extreme-precision, multidimensional quadrature on intricate integrands; thus, a highly parallel computation was required.


PACS

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

02.70.Wz Symbolic computation (computer algebra)

02.30.Gp Special functions

MSC

33F10 Symbolic computation (Gosper and Zeilberger algorithms, etc.) (See also 68W30)

82B20 Lattice systems (Ising, dimer, Potts, etc.) and systems on graphs

33Cxx Hypergeometric functions

Subjects

Mathematical physics

Computational physics

Statistical physics and nonlinear systems

Dates

Issue 40 (6 October 2006)

Received 2 June 2006, in final form 14 August 2006

Published 19 September 2006



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