Abstract
Let Vect(RP1) be the Lie algebra of smooth vector fields on RP1. In this paper, we classify aff(1)-invariant linear differential operators from Vect(RP1) to D-lambda,D-mu;nu vanishing on aff(1), where D-lambda,D-mu;nu := Homdiff (F-lambda circle times F-mu; F-nu) is the space of bilinear differential operators acting on weighted densities. This result allows us to compute the first differential aff(1)relative cohomology of Vect(RP1) with coefficients in D-lambda,D-mu;nu.