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Approximation of functions, measure of

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A quantitative expression for the error of an approximation. When the discussion is about the approximation of a function by a function \phi , the measure of approximation \mu (f, \phi ) is usually defined by the metric in a function space containing both f and \phi . For example, if f and \phi are continuous functions on a segment [a, b] , the uniform metric of C [a, b] is commonly used, i.e. one puts

\mu (f, \phi ) = \ \max _ {a \leq t \leq b } \ | f (t) - \phi (t) | .

If continuity of the approximated function is not guaranteed or if the conditions of the problem imply that it is important that f and \phi are close on [a, b] in an average sense, the integral metric of a space L _ {p} [a, b] may be used, putting

\mu (f, \phi ) = \ \int\limits _ { a } ^ { b } q (t) | f (t) - \phi (t) | ^ {p} \ dt,\ p > 0,

where q (t) is a weight function. The case p = 2 is most often used and is most convenient from a practical point of view (cf. Mean-square approximation of a function).

The measure of approximation may take into account only values of f and \phi in discrete points t _ {k} , k = 1 \dots n , of [a, b] , e.g.

\mu (f, \phi ) = \ \max _ {1 \leq k \leq n } \ | f (t _ {k} ) - \phi (t _ {k} ) | ,

\mu (f, \phi ) = \sum _ {k = 1 } ^ { n } q _ {k} | f (t _ {k} ) - \phi (t _ {k} ) | ^ {p} ,

where q _ {k} are certain positive coefficients.

One defines in an analogous way the measure of approximation of functions in two or more variables.

The measure of approximation of a function f by a family F of functions is usually taken to be the best approximation:

E (f, F) = \ \mu (f, F) = \ \inf _ {\phi \in F } \ \mu (f, \phi ).

The quantity

E ( \mathfrak M , F) = \ \mu ( \mathfrak M , F) = \ \sup _ {f \in \mathfrak M } \ \inf _ {\phi \in F } \ \mu (f, \phi )

is usually taken as the measure of approximation of a class \mathfrak M of functions f by functions \phi from a certain fixed set F . It characterizes the maximal deviation of functions in \mathfrak M from functions in F that are closest to them.

In general, when approximation in an arbitrary metric space X is considered, the measure of approximation \mu (x, u) of an element x by an element u ( a set F ) is the distance \rho (x, u) ( or \rho (x, F) ) between x and u ( or F ) in the metric of X .

References

[1] V.L. Goncharov, "The theory of interpolation and approximation of functions" , Moscow (1954) (In Russian)
[2] S.M. Nikol'skii, "Approximation of functions of several variables and imbedding theorems" , Springer (1975) (Translated from Russian)
[3] J.R. Rice, "The approximation of functions" , 1–2 , Addison-Wesley (1964–1968)

Comments

The measure of approximation is also called the error measure.

References

[a1] G.G. Lorentz, "Approximation of functions" , Holt, Rinehart & Winston (1966)
[a2] A. Pinkus, "n-widths in approximation theory" , Springer (1985) (Translated from Russian)
How to Cite This Entry:
Approximation of functions, measure of. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Approximation_of_functions,_measure_of&oldid=53270
This article was adapted from an original article by N.P. KorneichukV.P. Motornyi (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article