Stechkin's lemma

Stechkin's lemma is a result in the field of functional analysis and approximation theory, particularly concerning the properties of certain sequences of functions and their convergence. It is often referenced in the context of studying the approximation of functions in terms of series expansions and the behavior of polynomials. The lemma generally states conditions under which a sequence of functions (often approximating polynomials or Fourier series) converges uniformly to a continuous function.