  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.

High Energy Physics - Theory