1. E. Cohen, Unitary functions (mod r). Duke Math. J. 28, 475–485 (1961) and II. Publ. Math. Debrecen 9, 94–104 (1962)
2. E. Cohen, Arithmetical notes VIII. An asymptotic formula of A. Rényi. Proc. Am. Math. Soc. 13, 536–539 (1962)
3. E. Cohen, Some asymptotic formulas in the theory of numbers. Trans. Am. Math. Soc. 112, 214–227 (1964)
4. N.G. de Bruijn, Asymptotic Methods in Analysis, Bibliotheca Mathematica, vol. 4 (North-Holland Publishing, Amsterdam; P. Noordhoff Ltd., Groningen; Interscience Publishers, New York, 1958), xii+200 pp.
5. N.G. de Bruijn, On the number of integers $$\leqslant x$$ whose prime factors divide n. Illinois J. Math. 6, 137–141 (1962)