A perturbation result for bounded imaginary powers

We give a new proof of a perturbation result due to J. Prüss and H. Sohr 11]: if an operator A has bounded imaginary powers, then so does A+w (w ≧ 0). Instead of Mellin transform on which the proof in 11] is based, we use the functional calculus for sectorial operators developed in particular by A. McIntosh (8], 3] and 1]). It turns out that our method gives a more general result than the one used in 11].