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 A230606 #14 May 28 2019 15:37:17 %S A230606 2,3,5,7,11,13,17,19,20,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83, %T A230606 89,97,101,103,104,107,109,113,127,131,137,139,149,151,157,163,167, %U A230606 173,179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263 %N A230606 Numbers n such that sigma(n) = k*(n+1) for some integer k. %C A230606 Numbers n such that A108775(n) = floor(sigma(n) / n) = sigma(n) mod n = A054024(n). %C A230606 Union primes (A000040) and composite numbers A045768 (k = 1 for primes p, k = 2 for composite numbers). %H A230606 Jaroslav Krizek, <a href="/A230606/b230606.txt">Table of n, a(n) for n = 1..1000</a> %e A230606 20 is in sequence because sigma(20) = 42 = 2*21. %t A230606 Select[Range[300],Divisible[DivisorSigma[1,#],#+1]&] (* _Harvey P. Dale_, May 28 2019 *) %Y A230606 Cf. A000203(sigma(n)), A054024 (sigma(n) mod n), A108775. %Y A230606 Cf. A045768 (sigma(n) == 2 (mod n)). %K A230606 nonn %O A230606 1,1 %A A230606 _Jaroslav Krizek_, Nov 29 2013 %E A230606 Example clarified by _Harvey P. Dale_, May 28 2019