[1] G. H. Hardy and S. Ramanujan, The normal number of prime factors of a number n, Q. J. Math. 48 (1917) 76 − 92.
[2] E. G. H. Landau, Handbuch der Lehre von der Verteilung der Primzahlen, 3rd edn. Chelsea Publishing Company, New York, 1974.
[3] J. Lee, The second central moment of additive functions, Proc. Amer. Math. Soc. 114 (4) (1992) 887 − 895.
[4] J. B. Rosser and L. Schoenfeld, Approximate formulas for some functions of prime numbers, Illinois J. Math. 6 (1962) 64 − 94.