35G05 General theory of linear higher-order PDE
35G10 Initial value problems for linear higher-order PDE, linear
evolution equations
35G15 Boundary value problems for linear higher-order PDE
35G20 General theory of nonlinear higher-order PDE
35G25 Initial value problems for nonlinear higher-order PDE,
nonlinear evolution equations
35G30 Boundary value problems for nonlinear higher-order PDE
35G99 None of the above but in this section