Matrix Models, Quantum Penner Action and Two-Dimensional String Theory

A 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.