Residual (mathematics)
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In mathematics, a residual set is the complement of a meager set. A meager set is one which is the countable union of nowhere dense sets.
Also, loosely speaking, a residual is the error in a result. To be precise, suppose we want to find x such that
Given an approximation of x0 of x, the residual is
whereas the error is
If we do not know x, we cannot compute the error but we can compute the residual.
Residuals appear in many areas in mathematics, from iterative solvers such as the generalized minimal residual method, which seeks solutions to equations by systematically minimizing the residual, to statistics.
- Jonathan Richard Shewchuk. An Introduction to the Conjugate Gradient Method Without the Agonizing Pain, p. 6.


