The Buchstab function (or Buchstab's function) is the unique continuous function defined by the delay differential equation
In the second equation, the derivative at u = 2 should be taken as u approaches 2 from the right. It is named after Alexander Buchstab, who wrote about it in 1937.
Asymptotics The Buchstab function approaches rapidly as where is the Euler–Mascheroni constant. In fact,
where ρ is the Dickman function.(5.13), Jurkat and Richert 1965. In this paper the argument of ρ has been shifted by 1 from the usual definition. Also, oscillates in a regular way, alternating between extrema and zeroes; the extrema alternate between positive maxima and negative minima. The interval between consecutive extrema approaches 1 as u approaches infinity, as does the interval between consecutive zeroes.p. 131, Cheer and Goldston 1990.
Applications The Buchstab function is used to count rough numbers. If Φ(x, y) is the number of positive integers less than or equal to x with no prime factor less than y, then for any fixed u > 1,
Notes
References
"Buchstab Function", Wolfram MathWorld. Accessed on line Feb. 11, 2015. §IV.32, "On Φ(x,y) and Buchstab's function", Handbook of Number Theory I, József Sándor, Dragoslav S. Mitrinović, and Borislav Crstici, Springer, 2006, . "A differential delay equation arising from the sieve of Eratosthenes", A. Y. Cheer and D. A. Goldston, Mathematics of Computation 55 (1990), pp. 129–141. "An improvement of Selberg’s sieve method", W. B. Jurkat and H.-E. Richert, Acta Arithmetica 11 (1965), pp. 217–240.