Derivable mappings at unit operator on nest algebras

Let A be an operator subalgebra with the unit operator I in B(H). We say that a linear mapping φ from A into itself is a derivable mapping at I if φ(ST)=φ(S)T+Sφ(T) for any S,T∈A with ST = I. In this paper, we show the following main result: every strongly operator topology continuous derivable mapping at I on a nest algebra algN is an inner derivation.