Variational principle in the bag model

A variational principle describing the bag model is found. The principle lies in searching extremum of the action W = ∫sub(v) (lsub(ch)-B)dsup(4)x, where Lsub(ch) is the chromodynamical Lagrangian, with the supplementary condition of ant psipsi=0, which determines a dependence of the field variation on the boundary variation. The variational principle is applied to fermion field and to scalar field, and the boundary conditions for the field are obtained