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 A205525 #19 Feb 09 2021 02:44:15 %S A205525 1,6,12,18,20,24,28,40,56,88,104,120,180,196,224,234,240,360,368,420, %T A205525 464,496,540,600,650,672,780,992,1080,1344,1504,1872,1888,1890,1952, %U A205525 2016,2184,2352,2376,2688,3192,3276,3724,3744,4284,4320,4680,5292,5376,5624 %N A205525 Nonprime numbers k such that gcd(k, sigma(k)) == sigma(k) (mod k). %C A205525 Complement of primes (A000040) with respect to A205523. %e A205525 24 is in the sequence because gcd(24; sigma(24)=60) = (sigma(24)=60) mod 24 = 12. %t A205525 Select[Range[10000], ! PrimeQ[#] && Mod[GCD[#, DivisorSigma[1, #]] - DivisorSigma[1, #], #] == 0 &] (* _T. D. Noe_, Feb 03 2012 *) %o A205525 (PARI) isok(k) = if (!isprime(k), my(s=sigma(k)); Mod(gcd(k, s), k) == Mod(s, k)); \\ _Michel Marcus_, Feb 09 2021 %Y A205525 Cf. A000203, A000040, A009194, A054024, A205524, A205523. %K A205525 nonn %O A205525 1,2 %A A205525 _Jaroslav Krizek_, Jan 28 2012 %E A205525 Corrected by _T. D. Noe_, Feb 03 2012