本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 - Z. I2 U o7 J1 w; o1 {
8 D" k$ q/ L0 @* e0 J5 i: xIn 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. # \) r- o' [' f4 ]2 d' GIn 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.3 `2 G% A3 ]% S3 S, c: u
Transformations of functions is a somewhat more general concept, see operator.