本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 / h! J6 ?' E1 |5 K9 J4 i0 e' _" s$ D
In mathematics, a functional is traditionally a map from a vector space to the field underlying the vector space, which is usually the real numbers. In other words, it is a function that takes a vector as its argument or input and returns a scalar. Its use goes back to the calculus of variations where one searches for a function which minimizes a certain functional. A particularly important application in physics is to search for a state of a system which minimizes the energy functional.% U2 G; j0 l! H- }& Q" X
In functional analysis, the functional is also used in a broader sense as a mapping from an arbitrary vector space into the underlying scalar field (usually, real or complex numbers). A special kind of such functionals, linear functionals, gives rise to the study of dual spaces. 1 F6 Q4 h8 D% g. G( q9 G/ iTransformations of functions is a somewhat more general concept, see operator.