An Improvement of the Feynman Action in the Theory of Polaron. I

An improvement of the Feynman action is discussed by introducing two additional parameters to Feynman's original polaron theory. Both upper bounds of the ground state energy and corrections to the polaron mass are obtained for some values of the coupling constant. However, corrections are so small that Feynman's trial action is proved to be a very good description of the polaron state.