Bogoliubov, Parasiuk, and Stepanov develop R-operations demonstrating
self-consistency of perturbation theory in QFT.
N.N. Bogoliubov, O.S. Parasiuk, "Ueber die Multiplikation der Kausal
Funktionen in der Quantentheorie der Felder," Acta Mathematica 97
(1957): 227; "Umnozhenie prichinnykh funktsii pri nesovpadaiushchikh
argumentakh," Izv. Akad. Nauk SSSR ser. mat. 20 (1956): 843.
B.M. Stepanov, "Abstraktnaia teoriia R-operatsii," Izv. AN SSSR, ser.
mat. 27 (1963): 819.