Chebyshev system

Summary

A Chebyshev system, or Haar system, is a family of $n$ continuous functions whose nonzero linear combinations have at most $n - 1$ zeros. Polynomials of degree less than $n$ are the model case. Their span is called a Haar subspace. For Chebyshev systems, interpolation at any $n$ distinct points is uniquely solvable. By Haar's theorem, they are also the families for which best uniform approximation is always unique.