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 A206449 #22 Dec 16 2024 04:15:01 %S A206449 12,18,24,32,42,48,54,60,72,80,84,90,98,104,108,114,128,132,140,152, %T A206449 168,180,182,192,224,228,234,240,252,264,270,272,294,308,312,360,384, %U A206449 390,420,432,434,440,444,450,468,480,492,504,522,558,570,572,588,594,600 %N A206449 Values of sigma(p) of primes p such that sigma(p) = sigma(c) has a solution for some composite number c. %C A206449 Corresponding values of sigma(p) of primes p from A158913. %H A206449 Amiram Eldar, <a href="/A206449/b206449.txt">Table of n, a(n) for n = 1..10000</a> %H A206449 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp). %F A206449 a(n) = A158913(n) + 1. %t A206449 Union@ Select[ DivisorSigma[1, Select[ Range@ 100, !PrimeQ@# &]], PrimeQ[# - 1] &] (* _Robert G. Wilson v_, Feb 09 2012 *) %o A206449 (PARI) list(lim) = forprime(p = 1, lim, if(invsigmaNum(p+1) > 1, print1(p+1, ", "))); \\ _Amiram Eldar_, Dec 16 2024, using _Max Alekseyev_'s invphi.gp %Y A206449 Cf. A000203, A158913 (primes p such that there is a composite c with sigma(p)=sigma(c)), A206448 (values of sigma(c) of composite numbers c such that sigma(c) = sigma(d) has a solution for any other composite number d). %K A206449 nonn %O A206449 1,1 %A A206449 _Jaroslav Krizek_, Feb 07 2012