  An assignment problem is the optimization problem of finding, in an m by n
matrix of nonnegative real numbers, k entries, no two in the same row or
column, such that their sum is minimal. Such an optimization problem is called
a random assignment problem if the matrix entries are random variables. We give
a formula for the expected value of the optimal k-assignment in a matrix where
some of the entries are zero, and all other entries are independent
exponentially distributed random variables with mean 1. Thereby we prove the
formula 1+1/4+1/9+...+1/k^2 conjectured by G. Parisi for the case k=m=n, and
the generalized conjecture of D. Coppersmith and G. B. Sorkin for arbitrary k,
m and n.
