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 A259309 #21 Jun 29 2025 15:17:24 %S A259309 1,3,5,12,14,15,19,23,28,33,34,35,39,40,47,53,57,58,73,76,79,86,88,89, %T A259309 104,112,116,118,126,131,133,134,138,139,145,147,148,154,163,165,173, %U A259309 175,179,183,185,189,193,197,201,204,206,207,213,216,219,220,224 %N A259309 Numbers k such that 1 + sigma(k)^4 is prime. %H A259309 Robert Price, <a href="/A259309/b259309.txt">Table of n, a(n) for n = 1..1459</a> %H A259309 OEIS Wiki, <a href="https://oeis.org/wiki/Cyclotomic Polynomials at x=n, n! and sigma(n)">Cyclotomic Polynomials at x=n, n! and sigma(n)</a> %p A259309 with(numtheory): A259309:=n->`if`(isprime(1 + sigma(n)^4), n, NULL): seq(A259309(n), n=1..500); # _Wesley Ivan Hurt_, Jul 09 2015 %t A259309 Select[ Range[10000], PrimeQ[ 1 + DivisorSigma[1, #]^4] & ] %t A259309 Select[ Range[10000], PrimeQ[ Cyclotomic[8, DivisorSigma[1, #]]] &] %o A259309 (Magma) [n: n in [1..250] | IsPrime(1+SumOfDivisors(n)^4)]; // _Vincenzo Librandi_, Jun 24 2015 %o A259309 (PARI) is(n)=my(s=sigma(n)); isprime(s^4+1) \\ _Charles R Greathouse IV_, May 22 2017 %Y A259309 Cf. A000203, A259308, A259310. %K A259309 easy,nonn %O A259309 1,2 %A A259309 _Robert Price_, Jun 24 2015