本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 6 A3 Z9 z6 p, v. B 9 k7 h9 I# |4 Q6 z' bIn 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. ! ~' [% P8 P( } T9 aIn 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.: [% i' D4 T. g5 p/ H. b6 U
Transformations of functions is a somewhat more general concept, see operator.