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 A205523 #21 Aug 23 2023 08:41:31 %S A205523 1,2,3,5,6,7,11,12,13,17,18,19,20,23,24,28,29,31,37,40,41,43,47,53,56, %T A205523 59,61,67,71,73,79,83,88,89,97,101,103,104,107,109,113,120,127,131, %U A205523 137,139,149,151,157,163,167,173,179,180,181,191,193,196,197,199 %N A205523 Numbers k such that gcd(k, sigma(k)) == sigma(k) (mod k). %C A205523 Numbers m such that A009194(m) = gcd(m, A000203(m)) = A000203(m) mod m = A054024(m). %C A205523 Complement of A205524. Union of primes (A000040) and composite numbers from A205525. %H A205523 Ivan Neretin, <a href="/A205523/b205523.txt">Table of n, a(n) for n = 1..10000</a> %e A205523 Number 24 is in sequence because gcd(24, sigma(24)) = (sigma(24)=60) mod 24 = 12. %t A205523 Select[Range[300], Mod[GCD[#, DivisorSigma[1, #]] - DivisorSigma[1, #], #] == 0 &] %o A205523 (PARI) isok(n) = (gcd(n, sigma(n)) % n) == (sigma(n) % n); \\ _Michel Marcus_, Dec 22 2017 %Y A205523 Cf. A000203, A000040, A009194, A054024, A205524, A205525. %K A205523 nonn %O A205523 1,2 %A A205523 _Jaroslav Krizek_, Jan 28 2012 %E A205523 Corrected by _T. D. Noe_, Feb 03 2012