Physics of Scale Activities

1934
Fock functionals
Fock develops a method for describing a system with an indeterminate number of particles by means of a generating functional, a method now known as the method of Fock functionals. The first application (in quantum electrodynamics) of the apparatus of variational differentiation.

V. Fock, "Zur Quantenelektrodynamik," Phys. Zeits. Sowjetunion 6 (1934): 425-469.
--Karl Hall