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Sehwinger's variational principle has been used for a wide variety of problems involving wave motion in which it is desired to find the amplitude of a scattered wave in terms of the incident wave.** Sehwinger's method makes use of the fact that the amplitude of the scattered wave satisfies a variational principle. We shall indicate this variational principle briefly. Let us consider a vector space. We may define two different inner products, the Hermitian and symmetric inner product, in this vector space. The Hermitian inner product (a,b) of two vectors a and b is defined by the condition that

A note on the application of Schwinger’s variational principle to Dirac’s equation of the electron