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 A385115 #53 Aug 18 2025 06:28:56 %S A385115 1,3,9,13,31,43,81,121,235,1135,1245,1521,2019,2329,3573,11245,15571, %T A385115 37333,54471,70641 %N A385115 Numbers k such that 2^4 * 3^k - 1 is prime. %C A385115 All terms are odd, since if k were even, N = 2^4 * 3^k would be a perfect square and N - 1 could be factored as the difference of squares, hence not prime. %C A385115 a(21) > 10^5. - _Michael S. Branicky_, Aug 15 2025 %t A385115 Select[Range[4000], PrimeQ[16 * 3^# - 1] &] (* _Amiram Eldar_, Aug 15 2025 *) %o A385115 (Python) %o A385115 from gmpy2 import is_prime %o A385115 print([k for k in range(1, 4_000, 2) if is_prime(16 * 3**k - 1)]) %Y A385115 Cf. A005540, A005541, A387060. %K A385115 nonn,more %O A385115 1,2 %A A385115 _Ken Clements_, Aug 14 2025 %E A385115 a(17)-a(20) from _Michael S. Branicky_, Aug 15 2025