Special functions associated with automorphisms of the space of solutions to special double confluent Heun equation

Abstract

The family of quads of interrelated functions holomorphic on the universal cover of the complex plane without zero (for brevity, pqrs-functions), revealing a number of remarkable properties, is introduced. In particular, under certain conditions the transformations of the argument z of pqrs-functions represented by lifts of the replacements z Ø ´1{z, z Ø ´z, and z Ø 1{z prove to be equivalent to linear transformations with known coefficients. Pqrsfunctions are utilized for constructing of linear operators acting as automorphisms on the space of solutions to the special double confluent Heun equation (sDCHE). Earlier such symmetries were known to exist only in the case of integer value of one of the constant parameters. Then pqrs-functions appear as polynomials, a constructive algorithm of their computation being available. In the present work, the theory of discrete symmetries of the space of solutions to sDCHE is extended to the general case, apart from some natural exceptions.