本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 ( M" E B1 C! I w3 @ 2 @9 A6 T# g- v+ `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.. ]; g# F. X* Q' f: U
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. $ O1 s6 }1 W: m2 ^Transformations of functions is a somewhat more general concept, see operator.