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 A309548 #26 May 14 2023 02:33:29 %S A309548 2,3,4,5,6,11,13,21,29,31,37,170,180,214,234,265,362,369,10734,14318, %T A309548 19679,19876,39636,48784,62517,76225,77277,83629,85519,90649,92287 %N A309548 Numbers k such that sigma(k)! - 1 is prime, where sigma is A000203. %e A309548 2 is a term because sigma(2) = 3. 3! - 1 = 5, a prime. %e A309548 6 is a term because sigma(6) = 12. 12! - 1 = 479001599, a prime. %o A309548 (SageMath) %o A309548 [n for n in range(1,150) if is_prime(factorial(sigma(n))-1)] %o A309548 (PARI) isok(n) = isprime(sigma(n)!-1); \\ _Michel Marcus_, Aug 07 2019 %Y A309548 Cf. A000040, A000203, A002982, A055490, A184388. %K A309548 nonn,more %O A309548 1,1 %A A309548 _Hauke Löffler_, Aug 07 2019 %E A309548 a(12)-a(24) from _Daniel Suteu_, Aug 07 2019 %E A309548 a(25)-a(31) from _Amiram Eldar_, May 14 2023