Cohen, Graeme L.; Hagis, Peter Jr. On the number of prime factors of n if φ(n) divides n − 1. Nieuw Arch. Wiskd. III Series. 1980, 28: 177–185. ISSN 0028-9825. Zbl 0436.10002.
Hagis, Peter Jr. On the equation M·φ(n) = n − 1. Nieuw Arch. Wiskd. IV Series. 1988, 6 (3): 255–261. ISSN 0028-9825. Zbl 0668.10006.
Cohen, Graeme L.; Hagis, Peter Jr. On the number of prime factors of n if φ(n) divides n − 1. Nieuw Arch. Wiskd. III Series. 1980, 28: 177–185. ISSN 0028-9825. Zbl 0436.10002.
Hagis, Peter Jr. On the equation M·φ(n) = n − 1. Nieuw Arch. Wiskd. IV Series. 1988, 6 (3): 255–261. ISSN 0028-9825. Zbl 0668.10006.