Integral relations and new solutions to the double-confluent Heun equation

Integral relations and transformation rules are used to obtain, out of an asymptotic solution, a new group of four pairs of solutions to the double-confluent Heun equation. Each pair presents the same series coefficients but has solutions convergent in different regions of the complex plane. Integral relations are also established between solutions given by series of Coulomb wave functions. The Whittaker-Hill equation and another equation are studied as particular cases of both the double-confluent and the single-confluent Heun equations. Finally, applications for the Schr\"{o}dinger equation with certain potentials are discussed, mainly for quasi-exactly solvable potentials which lead to the above special equations.