Chebyshev subspaces and convergence of positive linear operators

A theorem of Korovkin states that a sequence of positive linear operators on C[a, 6] converges strongly to the identity if and only if convergence holds on a three-dimensional Chebyshev subspace of C[a, b]. We extend this theorem to include Chebyshev subspaces of arbitrary dimension and convergence to other positive linear operators. © 1973, American Mathematical Society.