本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 9 I+ {2 L Z* ]9 y C0 X# k$ c
4 f6 _2 j. n1 f8 A* s6 iIn 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.4 u( w, f; p7 h/ {3 ~! A
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.# Q" I# Q& H6 ], C. N, J1 T# E& r1 ]1 E
Transformations of functions is a somewhat more general concept, see operator.