1. F. Beukers , F. Luca , F. Oort, Power values of divisor sums, The American Mathematical Monthly 119, 5 (2012), 373–380.
2. L. E. Dickson, History of the Theory of Numbers, Vol. I, Divisibility and Primality, Chelsea Publishing, New York, 1966.
3. Euclid, Elements, Book IX, Proposition 36.
4. L. Euler, De numeris amicibilibus, Commentationes arithmeticae (in Latin, published posthumously), 2 (1849), 627–636.
5. T. Freiberg, Products of shifted primes simultaneously taking perfect power values, J. Aust. Math. Soc. (special issue dedicated to Alf van der Poorten) 92 (2012), 145–154.