Additive Biderivations of Incidence Algebras

We characterize all additive biderivations on the incidence algebra I(P,R) of a locally finite poset P over a commutative ring with unity R. By decomposing P into its connected chains, we prove that any additive biderivation splits uniquely into a sum of inner biderivations and extremal ones determined by chain components. In particular, when every maximal chain of P is infinite, all additive biderivations are inner.