The set of solutions to the string equation [P, Q] = 1 where P and Q are differential operators is described.It is shown that there exists oneto-one correspondence between this set and the set of pairs of commuting differential operators.This fact permits us to describe the set of solutions to the string equation in terms of moduli spa-ces of algebraic curves,however the direct description is much simpler. Some results are obtained for the superanalog to the string equation where P and Q are considered as super differential operators. It is proved that this equation is invariant with respect to Manin-Radul, Mulase-Rabin and Kac-van de Leur KP-hierarchies.
Let us consider ordinary differential operatorswhere ∂ = ∂ ∂x , a m (x) and b n (x) are formal power series with complex coefficients. The approach to non-critical string theory based on the consideration of matrix models [1]-[3] led to the problem of description of all pairs P, Q satisfying [P, Q] = 1 (see [4]). The equation [P, Q] = 1 where P and Q are differential operators is known therefore as string equation. We will assume that the orders p and h of differential operators P and Q have no common divisors and that Q is monic (i.e. the leading coefficient b h (x) is equal to 1). The set of all pairs (P, Q) obeying these conditions will be denoted by A p,h .Every monic operator Q = h r=1 b r (x)∂ r can be normalized by means of transformation Q →Q = e γ Qe −γ (i.e. one can make b h−1 = 0).If (P, Q) ∈ A p,h then (P ,Q) whereP = e γ P e −γ ,Q = e γ Qe −γ belongs to A p,h too. Therefore the study of the space A p,h can be reduced to the study of the space Q p,h consisting of pairs (P, Q) ∈ A p,h where Q is normalized.One of our aims is to describe the set Q p,h . We give the following description. Let us denote by M p,h the space of polynomial h × h matrices P = (P ij (u)) satisfying p = max 1≤j≤h (j − i + h deg P ij ) for every i. (Here i, j = 1, . . . , h, deg P ij denotes the degree of the polynomial P ij (u)).