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Most even amicable pairs have a sum which is divisible by 9; for instance, the sum of (220, 284) is 504.

This list is not known to be complete (564 might be a member). See A122726. - N. J. A. Sloane, Sep 17 2021

Up to the known 1593 sociable number cycles, 96.1% of the sociable number cycles satisfy this condition (up to the first 10 sociable number cycles: 40%; up to the first 100 sociable number cycles: 77%; up to the first 500 sociable number cycles: 92%, and up to the first 1000 sociable number cycles: 94.9%). So the conjecture here is that as the number of sociable number cycles increases, the percentage of the sums of the sociable number cycles divisible by 10 approaches 100%. Notice that the sums of amicable pairs are similarly often divisible by 10, but are not included here (see A291422).

Of the first 10^2, 10^6, and 10^10 positive integers, 17%, 40.8%, and 48.4%, respectively, are in the sequence (See Robert G. Wilson's table below). So the conjecture here is that if this trend continues then as the numbers approach infinity, the percentage of those numbers whose sum of divisors is divisible by 10 approaches 100%.

From David A. Corneth, Jan 01 2018: (Start)

If a(n) and m > 0 are coprime then a(n) * m is in the sequence.

A030433 is a subsequence.

Let p(n, d) be a prime ending in base-10 digit d and m != n gives p(m, d) != p(n, d). For p(n, 2) and p(n, 5) we must have n = 1 (one unique value). Then we could describe families of terms as factorizations in terms of these primes. 24 would give the family p(1, 2)^3 * p(1, 3).

For d coprime to 10 we could replace p(n, d) with p(n, 10 - d) and give another family of terms. Constructing terms could then be done by selecting primes for p(n, 1), p(n, 3), p(n, 7) and p(n, 9) from A030430, A030431, A030432 and A030433 respectively. (End)

From Robert G. Wilson v, Jan 03 2017: (Start)

The following array has for n powers of 10 and k is the number of integers <= 10^n which are == k (mod 10).

Array begins:

========================================================================================

n\k| 0 1 2 3 4 5 6 7 8 9

---|------------------------------------------------------------------------------------

_1 | 0 1 1 2 1 1 1 1 2 0

_2 | 17 6 16 4 21 2 12 4 17 1

_3 | 274 17 178 16 177 7 145 7 173 6

_4 | 3352 47 1690 44 1727 23 1498 27 1563 29

_5 | 37709 130 16012 144 16069 77 14649 87 15022 101

_6 | 408270 406 151936 424 152079 249 141527 293 144481 335

_7 | 4327266 1255 1451931 1341 1452017 787 1368110 923 1395278 1092

_8 | 45278675 3876 13963299 4206 13963783 2494 13268616 2998 13508556 3497

_9 | 469680154 11975 134976927 13185 134999718 7903 129084019 9684 131205200 11235

10 |4842279472 37545 1310133910 41349 1310381140 25017 1259051325 31136 1277983443 35663

... (End)

From Robert Israel, Jan 07 2018: (Start)

A number with prime factorization Product_j (p_j)^(e_j) is in the sequence if and only if

1) e_j is odd for some odd p_j, and

2) For some j, either p_j == 1 (mod 5) and e_j == 4 (mod 5), or p_j == 2 or 3 (mod 5) and e_j == 3 (mod 4), or p_j == 4 (mod 5) and e_j is odd.

By the strong form of Dirichlet's theorem, the product of 1-1/p for primes == 4 (mod 5) is 0, which implies that the asymptotic density of the sequence is 1.

(End)