On a Formation of Singularities of Solutions to Soliton Equations Represented by L,A,B-triples

We discuss the mechanism of formation of singularities of solutions to the Novikov-Veselov,modified Novikov-Veselov,and Davey-Stewartson Ⅱ(DSII)equations obtained by the Moutard type transformations.These equations admit the L,A,B-triple presentation,the generalization of the L,A-pairs for 2+1-soliton equations.We relate the blow-up of solutions to the non-conservation of the zero level of discrete spectrum of the L-operator.We also present a class of exact solutions,of the DSII system,which depend on two functional parameters,and show that all possible singularities of solutions to DSII equation obtained by the Moutard transformation are indeterminancies,i.e.,points when approaching which in different spatial directions the solution has different limits.