Singular sector of the Kadomtsev–Petviashvili hierarchy, δ̅ operators of nonzero index, and associated integrable systems

Abstract

Integrable hierarchies associated with the singular sector of the Kadomtsev– Petviashvili (KP) hierarchy, or equivalently, with δ̅ operators of nonzero index are studied. They arise as the restriction of the standard KP hierarchy to submanifolds of finite codimension in the space of independent variables. For higher δ̅ index these hierarchies represent themselves as families of multidimensional equations with multidimensional constraints. The δ̅ -dressing method is used to construct these hierarchies. Hidden Korteweg–de Vries, Boussinesq, and hidden Gelfand–Dikii hierarchies are considered, too.