Pell and Pell–Lucas polynomials are related to the well-known Chebyshev polynomials, named after the eminent Russian mathematician Pafnuty Lvovich Chebyshev (1821–1894). Just as the Pell polynomial family consists of two closely related sub-families {p n (x)} and {q n (x)}, the Chebyshev family is made up of two closely related sub-families {T n (x)} and {U n (x)}. (The letter T comes from the French transliteration, Tchebycheff or the German one, Tschebyscheff.) Chebyshev polynomials have applications to approximation theory, combinatorics, Fourier series, numerical analysis, geometry, graph theory, number theory, and statistics [184].