Spectral analysis of an even order differential operator
  Using the method of similar operators we study an even order differential operator with periodic, semiperiodic, and Dirichlet boundary conditions. We obtain asymptotic formulas for eigenvalues of this operator and estimates for its spectral decompositions and spectral projections. We also establish the asymptotic behavior of the corresponding analytic semigroup of operators. 
Dmitry M. Polyakov
math.SP