“…Then by a result of V. Peller [42,Theorem 7.3], we have that for any p ∈ (0, ∞): df ∈ L p if and only if f is in the Besov space B 1/p p,p (T). Peller's work has been extended to obtain even more precise relationships between f and the singular values of df , for example L. Gheorghe [22] found necessary and sufficient conditions on f to ensure that df is in an arbitrary Riesz-Fisher space. For more details from a quantised calculus perspective, see [9, Chapter 4, Section 3.α].…”