  We study the expansion coefficients of the tau function of the KP hierarchy.
If the tau function does not vanish at the origin, it is known that the
coefficients are given by Giambelli formula and that it characterizes solutions
of the KP hierarchy. In this paper, we find a generalization of Giambelli
formula to the case when the tau function vanishes at the origin. Again it
characterizes solutions of the KP hierarchy.
