Let F be a field of characteristic 0, Mn(F) the full matrix algebra over F, t the subalgebra of Mn(F) consisting of all upper triangular matrices. Any subalgebra of Mn(F) containing t is called a parabolic subalgebra of Mn(F). Let P be a parabolic subalgebra of Mn(F). A map φ on P is said to satisfy derivability if φ(x·y) = φ(x)·y+x·φ(y) for all x,y ∈ P, where φ is not necessarily linear. Note that a map satisfying derivability on P is not necessarily a derivation on P. In this paper, we prove that a map φ on P satisfies derivability if and only if φ is a sum of an inner derivation and an additive quasi-derivation on P. In particular, any derivation of parabolic subalgebras of Mn(F) is an inner derivation.
