本帖最后由 厚积薄发 于 2010-1-26 21:12 编辑 ' F3 l$ P6 [2 r1 i 3 ]7 ~% ?6 U7 y0 C- e% YIn 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. 2 @% d7 P) X: C& ^8 U0 y7 D @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.# s7 j9 D# B+ f5 j
Transformations of functions is a somewhat more general concept, see operator.