Paper detail

The periodic complex method in interpolation spaces

We discuss a question which relates to Calderon's complex interpolation method. More precisely, we will consider the so-called "periodic" complex interpolation method, studied by Peetre. (Which also corresponds to the spaces obtained by Calderon's construction using Banach space valued analytic functions, but defined on an annulus instead of the strip used by Calderon.) Cwikel showed that, using functions with a given period iλin the complex method mechanism, one obtains the same interpolation spaces as in the original version of the complex method, up to equivalence of norms. He also showed that one of the constants of this equivalence will, in some cases, "blow up" as λtends to zero. We will show that the equivalence constants tend to 1 as λtends to infinity. Intuitively, this means that when applying the complex method, it makes a very small difference if one restricts oneself to periodic functions, provided that the period is very large (or the corresponding annulus is very thin).