“…Furthermore, we set which satisfies the hypothesis The operator A is an infinitesimal generator of a semigroup of contractions in L 2 (0,1). The spectrum of A is given by the simple eigenvalues λ j =− π 2 ( j −1) 2 , j ∈ IN * and associated to the eigenfunctions defined by ϕ 1 ( x )=1 and (see, e.g., [13]). Taking the compact operator of the form The family ( ϕ j ) j ≥ 1 is an orthonormal basis of L 2 (0,1) and the solution of the system () can be written as: The operator A generates the semigroup of contractions S ( t ) defined by and we have …”