The operator function should be static and it should be a member of the containing type. The following code snippet illustrates how a typical operator function would look like.

Function spaces and operator theory form a rich and interconnected subdiscipline of modern analysis, embracing a variety of spaces that measure function regularity and oscillatory behaviour ...

H. Bart, Poles of the Resolvent of an Operator Function, Proceedings of the Royal Irish Academy. Section A: Mathematical and Physical Sciences, Vol. 74 (1974), pp. 169-184 ...