where A = mc ∫ n e d τ, and B = √ m c + emc ∫ Vn e d τ, and V (τ 0) is a value of potential at some point, specified by support of variation function δn, which is supposed to be infinitesimal. To advance toward Lagrange equation, we equate functional derivative to zero and after simple algebraic manipulations arrive to the following equation: