For the case of the generalized Laplacian, as well as regularity at zero, see Berline, Getzler & Vergne (2004, Proposition 9.35). For the general case of an elliptic pseudodifferential operator, see Seeley (1967). Seeley, R. T. (1967), "Complex powers of an elliptic operator", Singular Integrals (Proc. Sympos. Pure Math., Chicago, Ill., 1966), Providence, R.I.: American Mathematical Society, pp. 288–307, MR0237943