# Approximation of functions, measure of

A quantitative expression for the error of an approximation. When the discussion is about the approximation of a function $ f $
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