Physics of Scale Activities

1956 - 1963
R-operations
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.
--Karl Hall