Abstract
There are various formulations of quantum field theory, differing in the form of a basic quantity, viz. the transition amplitude. In the most commonly used operator approach, the transition amplitude is expressed as the vacuum expectation value of the product of particle creation and annihilation operators. These operators obey certain commutation relations (cf. Sect. 1.1). Another formulation is based on expressing the transition amplitude in terms of path integrals over the fields. In studying the gauge fields, the path-integral formalism has proven to be the most convenient.
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