Sociable number

As an example, the number 1,264,460 is a sociable number whose cyclic aliquot sequence has a period of 4:

The sum of the proper divisors of ${\displaystyle 1264460}$ (${\displaystyle =2^{2}\cdot 5\cdot 17\cdot 3719}$) is

1 + 2 + 4 + 5 + 10 + 17 + 20 + 34 + 68 + 85 + 170 + 340 + 3719 + 7438 + 14876 + 18595 + 37190 + 63223 + 74380 + 126446 + 252892 + 316115 + 632230 = 1547860,

the sum of the proper divisors of ${\displaystyle 1547860}$ (${\displaystyle =2^{2}\cdot 5\cdot 193\cdot 401}$) is

1 + 2 + 4 + 5 + 10 + 20 + 193 + 386 + 401 + 772 + 802 + 965 + 1604 + 1930 + 2005 + 3860 + 4010 + 8020 + 77393 + 154786 + 309572 + 386965 + 773930 = 1727636,

the sum of the proper divisors of ${\displaystyle 1727636}$ (${\displaystyle =2^{2}\cdot 521\cdot 829}$) is

1 + 2 + 4 + 521 + 829 + 1042 + 1658 + 2084 + 3316 + 431909 + 863818 = 1305184, and

the sum of the proper divisors of ${\displaystyle 1305184}$ (${\displaystyle =2^{5}\cdot 40787}$) is

1 + 2 + 4 + 8 + 16 + 32 + 40787 + 81574 + 163148 + 326296 + 652592 = 1264460.

The following categorizes all known sociable numbers as of July 2018 by the length of the corresponding aliquot sequence:

It is conjectured that if n is congruent to 3 modulo 4 then there is no such sequence with length n.

The 5-cycle sequence is: 12496, 14288, 15472, 14536, 14264

The only known 28-cycle is: 14316, 19116, 31704, 47616, 83328, 177792, 295488, 629072, 589786, 294896, 358336, 418904, 366556, 274924, 275444, 243760, 376736, 381028, 285778, 152990, 122410, 97946, 48976, 45946, 22976, 22744, 19916, 17716 (sequence A072890 in the OEIS). It was discovered by Ben Orlin.

These two sequences provide the only sociable numbers below 1 million (other than the perfect and amicable numbers).

The aliquot sequence can be represented as a directed graph, ${\displaystyle G_{n,s}}$, for a given integer ${\displaystyle n}$, where ${\displaystyle s(k)}$ denotes the sum of the proper divisors of ${\displaystyle k}$.[5] Cycles in ${\displaystyle G_{n,s}}$ represent sociable numbers within the interval ${\displaystyle [1,n]}$. Two special cases are loops that represent perfect numbers and cycles of length two that represent amicable pairs.

It is conjectured that as the number of sociable number cycles with length greater than 2 approaches infinity, the proportion of the sums of the sociable number cycles divisible by 10 approaches 1 (sequence A292217 in the OEIS).

• H. Cohen, On amicable and sociable numbers, Math. Comp. 24 (1970), pp. 423–429

• A list of known sociable numbers
• Extensive tables of perfect, amicable and sociable numbers
• Weisstein, Eric W. "Sociable numbers". MathWorld.
• A003416 (smallest sociable number from each cycle) and A122726 (all sociable numbers) in OEIS