Variation of a functional

first variation

A generalization of the concept of the differential of a function of one variable. It is the principal linear part of the increment of the functional in a certain direction; it is employed in the theory of extremal problems to obtain necessary and sufficient conditions for an extremum. This was the meaning of the term "variation of a functional" imparted to it as early as 1760 by J.L. Lagrange . He considered, in particular, the functionals of the classical calculus of variations of the form (1)

If a given function is replaced by and the latter is substituted in the expression for , one obtains, assuming that the integrand is continuously differentiable, the following equation: (2)

where as . The function is often referred to as the variation of the function , and is sometimes denoted by . The expression , which is a functional with respect to the variation , is said to be the first variation of the functional and is denoted by . As applied to the functional (1), the expression for the first variation has the form (3)

where A necessary condition for an extremum of the functional is that the first variation vanishes for all . In the case of the functional (1), a consequence of this necessary condition and the fundamental lemma of variational calculus (cf. du Bois-Reymond lemma) is the Euler equation: A method similar to (2) is also used to determine variations of higher orders (see, for example, Second variation of a functional).

The general definition of the first variation in infinite-dimensional analysis was given by R. Gâteaux in 1913 (see Gâteaux variation). It is essentially identical with the definition of Lagrange. The first variation of a functional is a homogeneous, but not necessarily linear functional. The usual name under the additional assumption that the expression is linear and continuous with respect to is Gâteaux derivative. Terms such as "Gâteaux variation" , "Gâteaux derivative" , "Gâteaux differential" are more frequently employed than the term "variation of a functional" , which is reserved for the functionals of the classical variational calculus .