“…Regard (3) as the form (NH), where X = L 2 (0, π), M = I − ζΔ, L 0 = −kΔ and B(t) ≡ 0. In [7] the operator M is degenerate. In particular, for N = 1 and ζ = −1/n 2 , associated with the boundary condition u(0, t) = u(π, t) = 0, the equation satisfies N (M ) = span{sin nx}.…”