“…In (27), (a m 1 ,· ) l and (a ·,m 2 ) l are the symbols of the complex powers of the operators a m 1 ,· (x 1 , x 2 , ξ 1 , D 2 ) and a ·,m 2 (x 1 , x 2 , D 1 , ξ 2 ). In order to obtain the terms in (26), (27), (28), we notice that the constant τ in (22) is arbitrary and the Laurent coefficients clearly do not change if we change the partition of R n 1 × R n 2 , therefore we can let τ tend to infinity. In this way both the fourth and fifth integral in (22) vanish, due to the continuity of the integral w.r.t.…”