本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 : S9 F$ u/ x5 y9 f# P9 ^2 g- o+ \/ ?/ c: b6 q. ~" Q
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.6 I; Z, J7 l+ a8 H" g( r: |
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. ( j% l# t* o2 A/ L% QTransformations of functions is a somewhat more general concept, see operator.