  We study a generalization of genus-zero $r$-spin theory in which exactly one
insertion has a negative-one twist, which we refer to as the "closed extended"
theory, and which is closely related to the open $r$-spin theory of Riemann
surfaces with boundary. We prove that the generating function of genus-zero
closed extended intersection numbers coincides with the genus-zero part of a
special solution to the system of differential equations for the wave function
of the $r$-th Gelfand-Dickey hierarchy. This parallels an analogous result for
the open $r$-spin generating function in the companion paper to this work.
