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. 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 numberâfor 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. 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. 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 :the sum of the proper divisors of 1305184 (=2^5\cdot40787) is: ::1 + 2 + 4 + 8 + 16 + 32 + 40787 + 81574 + 163148 + 326296 + 652592 = 1264460. the following categorizes all known sociable numbers as of april 2013 by the length of the corresponding aliquot sequence: {| align="center" border="1" cellpadding="4" |- bgcolor="#a0e0a0" align="center" !sequence length !number of sequences |- align="center" | 1 (perfect) | 48 |- align="center" | 2 (amicable) |11,994,387 |- align="center" |4 |210 |-align="center" |5 |1 |- align="center" |6 |5 |- align="center" |8 |3 |- align="center" |9 |1 |- align="center" |28 |1 |} ==references== *p. poulet, #4865, l'intermÃ©diaire des mathÃ©maticiens 25 (1918), pp. 100-101. *h. cohen, on amicable and sociable numbers, math. comp. 24 (1970), pp. 423-429 == external links == *[http://djm.cc/sociable.txt a list
Divisor_function Integer_sequences Number_theory