Best approximation in the mean
The best approximation of a function $x$ by functions $u$ from a fixed set $F$ when the measure (error) of approximation is expressed in terms of an integral metric (see Best approximation; Approximation in the mean).
Best approximation in the mean. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Best_approximation_in_the_mean&oldid=31677