Editing Fractional calculus (section)

==Functional calculus==
In the context of [[functional analysis]], functions {{math|''f''(''D'')}} more general than powers are studied in the [[functional calculus]] of [[spectral theorem|spectral theory]]. The theory of [[pseudo-differential operator]]s also allows one to consider powers of {{mvar|D}}. The operators arising are examples of [[singular integral operator]]s; and the generalisation of the classical theory to higher dimensions is called the theory of [[Riesz potential]]s. So there are a number of contemporary theories available, within which ''fractional calculus'' can be discussed. See also [[Erdélyi–Kober operator]], important in [[special function]] theory {{harv|Kober|1940}}, {{harv|Erdélyi|1950–1951}}.