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 A283515 #22 Apr 09 2025 15:02:42 %S A283515 2,3,4,16,19,31,7547 %N A283515 Numbers k such that sigma(k^(k-1)) is a prime. %C A283515 sigma(k) is the sum of the divisors of k (A000203). %C A283515 Numbers k such that A000203(A000169(k)) is a prime. %C A283515 a(8) > 10^4. %C A283515 Corresponding values of k^(k-1): 2, 9, 64, 1152921504606846976, ... %C A283515 Corresponding values of sigma(k^(k-1)): 3, 13, 127, 2305843009213693951, ... %C A283515 Subsequence of A280257 (numbers k such that tau(k^(k-1)) is prime). %C A283515 Prime terms are in A088790. %C A283515 For k < 1000, sigma(k^(k+1)) is prime only for k = 5: sigma(5^6) = sigma(15625) = 19531 (prime). %e A283515 sigma(4^3) = sigma(64) = 127 (prime). %t A283515 fQ[n_] := PrimeQ[DivisorSigma[1, n^(n - 1)]]; Select[Range@1000, fQ] (* _Robert G. Wilson v_, Mar 10 2017 *) %o A283515 (Magma) [n: n in [1..500] | IsPrime(SumOfDivisors(n^(n-1)))]; %o A283515 (PARI) isok(n) = isprime(sigma(n^(n-1))); \\ _Michel Marcus_, Mar 10 2017 %Y A283515 Cf. A000169, A000203, A088790, A280257. %K A283515 nonn,more %O A283515 1,1 %A A283515 _Jaroslav Krizek_, Mar 10 2017 %E A283515 a(7) from _Giovanni Resta_, Mar 10 2017