. "adjoint" . . . "differential" . . . "operator" . . . . . . "differential operators" . "differential operator" . . "Differential operator" . "intrinsic differential operators" . "linear differential operator" . "Heaviside notation" . . "ring of differential operators in many variables" . . . . . "differentiation operator" . "Differential Operators" . "differentiation" . "Differential operator" .