Loops versus Matrices - The nonperturbative aspects of noncritical string

The nonperturbative aspects of string theory are explored for non-critical string in two distinct formulations: loop equations and matrix models. The effects corresponding to D-brane in these formulations are especially investigated in detail. It is shown that matrix models can universally yield a definite value of the chemical potential for an instanton while loop equations can not. This implies that string theory may not be nonperturbatively formulated solely in terms of closed strings.