A mixed initial and
boundary value problem is considered for a partial differential equation of
the form Mut(x,t) + Lu(x,t) = 0, where M and L are elliptic differential
operators of orders 2 m and 2 l, respectively, with m ≦ l. The existence and
uniqueness of a strong solution of this equation in H0l(G) is proved by semigroup
methods.