In mathematics, sociable numbers are numbers whose aliquot sums form a cyclic sequence that begins and ends with the same number.
They are generalizations of the concepts of amicable numbers and perfect numbers.
The first two sociable sequences, or sociable chains, were discovered and named by the Belgian mathematician Paul Poulet in 1918.P. Poulet, #4865, L'Intermédiaire des Mathématiciens 25 (1918), pp.
100–101.
(The full text can be found at ProofWiki: Catalan-Dickson Conjecture.)
In a set of sociable numbers, each number is the sum of the proper factors of the preceding number, i.e., the sum excludes the preceding number itself.
For the sequence to be sociable, the sequence must be cyclic and return to its starting point.
The period of the sequence, or order of the set of sociable numbers, is the number of numbers in this cycle.
If the period of the sequence is 1, the number is a sociable number of order 1, or a perfect number—for example, the proper divisors of 6 are 1, 2, and 3, whose sum is again 6.
A pair of amicable numbers is a set of sociable numbers of order 2.
There are no known sociable numbers of order 3, and searches for them have been made up to 5 \times 10^7 as of 1970.
It is an open question whether all numbers end up at either a sociable number or at a prime (and hence 1), or, equivalently, whether there exist numbers whose aliquot sequence never terminates, and hence grows without bound.
Example
An example with period 4:
The sum of the proper divisors of 1264460 (=2^2\cdot5\cdot17\cdot3719) 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 1547860 (=2^2\cdot5\cdot193\cdot401) 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 1727636 (=2^2\cdot521\cdot829) is
1 + 2 + 4 + 521 + 829 + 1042 + 1658 + 2084 + 3316 + 431909 + 863818 = 1305184, and
the sum of the proper divisors of 1305184 (=2^5\cdot40787) is
1 + 2 + 4 + 8 + 16 + 32 + 40787 + 81574 + 163148 + 326296 + 652592 = 1264460.
List of known sociable numbers
The following categorizes all known sociable numbers  by the length of the corresponding aliquot sequence:
It is conjectured that if n is congruent to 3 modulo 4 then there are 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. .
These two sequences provide the only sociable numbers below 1 million (other than the perfect and amicable numbers).
Searching for sociable numbers
The aliquot sequence can be represented as a directed graph, G_{n,s}, for a given integer n, where s(k) denotes the sum of the proper divisors of k.
Cycles in G_{n,s} represent sociable numbers within the interval [1,n].
Two special cases are loops that represent perfect numbers and cycles of length two that represent amicable pairs.
Conjecture of the sum of sociable number cycles
It is conjectured that as the number of sociable number cycles with length greater than 2 approaches infinity, the percentage of the sums of the sociable number cycles divisible by 10 approaches 100%. .
References
H. Cohen, On amicable and sociable numbers, Math.
Comp.
24 (1970), pp.
423–429
External links
A list of known sociable numbers
Extensive tables of perfect, amicable and sociable numbers
A003416 (smallest sociable number from each cycle) and A122726 (all sociable numbers) in OEIS
