This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A306531 #16 Jul 26 2025 03:14:23 %S A306531 4,8,9,16,25,27,32,49,64,77,81,121,125,128,169,243,256,289,343,361, %T A306531 512,529,611,625,729,841,961,1024,1073,1331,1369,1681,1849,2033,2048, %U A306531 2187,2197,2209,2401,2809,3125,3481,3721,4096,4489,4913,5041,5293,5329,6031,6241 %N A306531 Composite numbers k such that the sum of their aliquot parts divides k-1. %H A306531 Amiram Eldar, <a href="/A306531/b306531.txt">Table of n, a(n) for n = 1..10000</a> %e A306531 Aliquot parts of 77 are 1, 7, 11 and 78/(1+7+11) = 76/19 = 4. %p A306531 with(numtheory): P:=proc(n) if not isprime(n) and frac((n-1)/(sigma(n)-n))=0 then n; fi; end: seq(P(i),i=2..6241); %t A306531 q[k_] := !PrimeQ[k] && Divisible[k-1, DivisorSigma[1, k]-k]; Select[Range[2, 6500], q] (* _Amiram Eldar_, Jul 26 2025 *) %o A306531 (PARI) isok(n) = (n!=1) && !isprime(n) && !frac((n-1)/(sigma(n)-n)); \\ _Michel Marcus_, Feb 28 2019 %Y A306531 Union of A059047 and A246547. %Y A306531 Cf. A203966, A306532. %K A306531 nonn,easy %O A306531 1,1 %A A306531 _Paolo P. Lava_, Feb 22 2019