Let be integrable in , let be of bounded variation in , let denote the least upper bound
of in , and let denote the total variation of in . Given the function
See also Jackson's Theorem, Legendre Series
References
Picone, M. Appunti di Analise Superiore. Naples, Italy,, p. 260, 1940.
Sansone, G. Orthogonal Functions, rev. English ed. New York: Dover, pp. 203-205, 1991.