Why are all dualities conformal? Theory and practical consequences

We relate duality mappings to the "Babbage equation" F(. F(. z)). =. z, with F a map linking weak- to strong-coupling theories and demonstrate that, under fairly general conditions, F may only be a specific conformal transformation of the fractional linear type. This general result has enormous practical consequences. For example, one can establish that weak- and strong-coupling series expansions of arbitrarily large finite size systems are trivially related, i.e., after generating one of those series the other is automatically determined through a set of linear constraints between the series coefficients. This latter relation partially solves or, equivalently, localizes the computational complexity of evaluating the series expansion to a simple fraction of those coefficients. As a bonus, those relations also encode non-trivial equalities between different geometric constructions in general dimensions, and connect derived coefficients to polytope volumes. We illustrate our findings by examining various models including, but not limited to, ferromagnetic and spin-glass Ising, and Ising gauge type theories on hypercubic lattices in 1. <. D<. 9 dimensions.