2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/149279A very elementary model of a single positive hermitian random matrix coupled to an external matrix is defined and studied. Expanding the exact effective action around its classical solution leads to the ``quantum Penner action'', from which a rich structure of correlation functions is obtained. These are shown to be equal to the all-orders perturbative expansion of tachyon amplitudes in the two-dimensional string at self-dual radius.Latex file, 8 pages. Based on a talk given by the second author at the NATO ASI, Cargese, July 11-25 1995High Energy Physics - Theorytext