On the constrained KP hierarchy II
  A constrained KP hierarchy is discussed that was recently suggested by Aratyn et al. and by Bonora et al. This hierarchy is a restriction of the KP to a submanifold of operators which can be represented as a ratio of two purely differential operators of prescribed orders. Explicit formulas for action of vector fields on these two differential operators are written which gives a new description of the hierarchy and provides a new, more constructive, proof of compatibility of the constraint with the hierarchy. Also the Poisson structure of the constrained hierarchy is discussed. 
L. A. Dickey