EN
For a holomorphic function ψ defined on a sector we give a condition implying the identity
$(X,𝒟(A^{α}))_{θ,p} = {x ∈ X | t^{-θ Re α} ψ(tA) ∈ L⁎^{p}((0,∞);X)}$
where A is a sectorial operator on a Banach space X. This yields all common descriptions of the real interpolation spaces for sectorial operators and allows easy proofs of the moment inequalities and reiteration results for fractional powers.