Descriptions of some K functionals for three spaces and reiteration

We prove some equivalence formulas for the K functional for three spaces. I n particular we calculate K(t, s, f, L-p0, L-p1, L-p2). We also give an int egral formula connecting the K functional for triples of the type (A(0), A( 1), A(2)), where A(1) is of class C(theta, A(0), A(2)), with the K function al for the pair (A(0), A(2)). This formula is used to prove a reiteration f ormula for interpolation of the triple (A(0), A(1), A(2)) when A(i) is of c lass C(theta(i), X, Y).