Weighted Identities for the Solutions of Generalized Korteweg–de Vries Equations

Consider the Korteweg–de Vries equation $u_t+u_{xxx}+uu_{x}=0$ and its generalization $u_t+u_{xxx}+f(u)_{x}=0$. For the solutions of these equations, weighted identities (differential and integral) are obtained. These identities make it possible to establish the blow-up (in finite time) of the solutions of certain boundary-value problems.