2-DIMENSIONAL YANG-MILLS THEORIES ARE STRING THEORIES

We show that two-dimensional SO(N) and Sp(N) Yang-Mills theories witho ut fermions can be interpreted as closed string theories. The terms in the 1/N expansion of the partition function on an orientable or nonor ientable manifold M can be associated with maps from a string worldshe et onto M. These maps are unbranched and branched covers of M with an arbitrary number of infinitesimal worldsheet cross-caps mapped to poin ts in M. These string theories differ from SU(N) Yang-Mills string the ory in that they involve odd powers of 1/N and require both orientable and nonorientable worldsheets.