On a three-dimensional symmetric Ising tetrahedron, and contributions to the theory of the dilogarithm and Clausen functions

Perturbative quantum field theory for the Ising model at the three-loop level yields a tetrahedral Feynman diagram C(a, b) with masses a and b and four other lines with unit mass. The completely symmetric tetrahedron CTet ≡ C(1, 1) has been of interest from many points of view, with several representations and conjectures having been given in the literature. We prove a conjectured exponentially fast convergent sum for C(1, 1), as well as provide further analytic support for an empirical relation for C(1, 1) as a remarkable difference of Clausen function values. Our presentation includes Propositions extending the theory of the dilogarithm Li2 and Clausen Cl2 functions, as well as their relation to other special functions of mathematical physics. The results strengthen connections between Feynman diagram integrals, volumes in hyperbolic space, number theory, and special functions and numbers, specifically including dilogarithms, Clausen function values, and harmonic numbers.