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 A158913 #20 Dec 16 2024 01:59:11 %S A158913 11,17,23,31,41,47,53,59,71,79,83,89,97,103,107,113,127,131,139,151, %T A158913 167,179,181,191,223,227,233,239,251,263,269,271,293,307,311,359,383, %U A158913 389,419,431,433,439,443,449,467,479,491,503,521,557,569,571,587,593,599 %N A158913 Primes p such that there is a composite c with sigma(p) = sigma(c). %C A158913 See A158914 for the sequence for sigma_2. %H A158913 Donovan Johnson, <a href="/A158913/b158913.txt">Table of n, a(n) for n = 1..10000</a> %H A158913 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp). %t A158913 tp=DivisorSigma[1,Select[Range[1000],PrimeQ]]; tc=DivisorSigma[1,Select[Range[1000],!PrimeQ[ # ]&]]; Intersection[tp,tc]-1 %o A158913 (Sage) [sigma(n)-1 for n in (2..600) if is_prime(sigma(n)-1) and n<sigma(n)-1<600] # _Giuseppe Coppoletta_, Dec 22 2014 %o A158913 (PARI) is(p) = isprime(p) && invsigmaNum(p+1) > 1; \\ _Amiram Eldar_, Dec 16 2024, using _Max Alekseyev_'s invphi.gp %Y A158913 Cf. A000203, A158914. %K A158913 nonn %O A158913 1,1 %A A158913 _T. D. Noe_, Mar 30 2009