Wikipedia says that Riesz proved the Riesz-Thorin theorem in 1926 without using any complex methods. Does anyone know where the original proof can be found?
Wikipedia seems to be wrong about the year. The full reference is:
Marcel Riesz: Sur les maxima des formes bilineaires et sur les fonctionelles lineaires. Acta Mathematica, Vol. 49 (1927), pp. 465-497.
It can also be found in M. Riesz's Collected Papers.