Abstract. Consider a classical elliptic pseudodifferential operator P on R n of order 2a (0 < a < 1) with even symbol. For example, P = A(x, D) a where A(x, D) is a secondorder strongly elliptic differential operator; the fractional Laplacian (−∆) a is a particular case. For solutions u of the Dirichlet problem on a bounded smooth subset Ω ⊂ R n , we show an integration-by-parts formula with a boundary integral involving (d −a u)| ∂Ω , where d(x) = dist(x, ∂Ω). This extends recent results of Ros-Oton, Serra and Valdinoci, to operators that are x-dependent, nonsymmetric, and have lower-order parts. We also generalize their formula of Pohozaev-type, that can be used to prove unique continuation properties, and nonexistence of nontrivial solutions of semilinear problems. An illustration is given with P = (−∆ + m 2 ) a . The basic step in our analysis is a factorization of P , P ∼ P − P + , where we set up a calculus for the generalized pseudodifferential operators P ± that come out of the construction.