Variational method
Variational method is the mathematical field of dealing with the functional, and the general calculus of the processing function. For example, such a function can be constructed by the integral of the unknown function and its derivative. The variational method eventually seeks an extreme function: they make the function obtain a maximum or a minimum. Some of the classic questions on the curve are expressed in this form: one example is the steepest descent line, and a particle along gravity can reach a point that is not directly below it at point A in the shortest time. In all curves from A to B, you must minimize the expression that represents the falling time. The key theorem of variational method is the Euler-Lagrangian equation. It corresponds to the critical point of the functional. When looking for the maximum and minimum values of a function, a small change in the analysis of a solution gives an approximation of the first order. It can not tell is to find the maximum or minimum (or not). ...