In approximation theory, a Haar space or Chebyshev space is a finite-dimensional subspace of , where is a compact space and either the real numbers or the complex numbers, such that for any given there is exactly one element of that approximates "best", i.e. with minimum distance to in supremum norm.
- Shapiro, Harold (1971). Topics in Approximation Theory. Springer. pp. 19–22. ISBN 3-540-05376-X.