A mapping $f$ of an arbitrary set $X$ into the set $\mathbb R$ of real numbers or the set $\mathbb C$ of complex numbers. If $X$ is endowed with the structure of a vector space, a topological space or an ordered set, then there arise the important classes of linear, continuous and monotone functionals, respectively (cf. Linear functional; Continuous functional; Monotone mapping).
|||A.N. Kolmogorov, S.V. Fomin, "Elements of the theory of functions and functional analysis" , 1–2 , Graylock (1957–1961) (Translated from Russian)|
Functional. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Functional&oldid=29314