Quantum field theory
Quantum field theory is a generalization of quantum mechanics. Krivchenkov believed that people need to understand at least the non–relativistic theory. He almost asked students to excuse him for the field theory, where only perturbation theory can be offered. Not only do the perturbation theory series diverge, but each perturbational term also in some sense is infinite, and a special renormalization of the interaction constant is required to give the result the physical sense.