Towards a sufficient criterion for collapse in 3D Euler equations
  A sufficient integral criterion for a blow-up solution of the Hopf equations (the Euler equations with zero pressure) is found. This criterion shows that a certain positive integral quantity blows up in a finite time under specific initial conditions. Blow-up of this quantity means that solution of the Hopf equation in 3D can not be continued in the Sobolev space $H^2({\cal R}^3)$ for infinite time. 
E.A. Kuznetsov