After Weierstrass and Runge, many mathematicians (in particular Walsh, Keldysh, and Lavrentyev) had been working on the same problem.
Similar to the Feynman diagrams in quantum field theory, these diagrams are an extension of Keldysh's technique for nonequilibrium processes in fluid dynamics.
Mstislav Rostropovich | Mstislav Keldysh | Mstislav Dobuzhinsky | Mstislav |