A friendly number is a positive integer that is a member of a friendly pair or a higher-order
friendly n-tuple. Numbers that are not friendly
are said to be solitary. There are some numbers
that can easily be proved to be solitary, but
the status of numbers 10, 14, 15, 20, and many others remains unknown (Hickerson
2002). The numbers known to be friendly are given by 6, 12, 24, 28, 30, 40, 42, 56,
60, ... (OEIS A074902).
Anderson, C. W. and Hickerson, D. Problem 6020. "Friendly Integers." Amer. Math. Monthly84, 65-66, 1977.Hickerson,
D. "Re: friendly/solitary numbers [was: typos]." seqfan@ext.jussieu.fr.
19 Sep 2002. /archive/hickerson_2002-09-19.Pollack,
P. and Pomerance, C. "Some Problems of Erdős on the Sum-of-Divisors Function."
Trans. Amer. Math. Soc. Ser. B3, 1-26, 2016. https://doi.org/10.1090/btran/10.Sloane,
N. J. A. Sequences A050972, A050973,
and A074902 in "The On-Line Encyclopedia
of Integer Sequences."
