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 A297362 #14 Aug 26 2023 03:44:07 %S A297362 5,9,21,23,33,47,51,73,85,89,93,129,167,217,223,263,315,341,381,585, %T A297362 819,1057,1365,3591,3855,4681,4871,5461,6141,6223,6719,7487,8193, %U A297362 11447,13107,13367,13797,14329,16513,18631,21845,24573,25575,26431,33825,37449 %N A297362 Numbers k such that (2^ord(2, k) - 1)/k is prime, where ord(2, k) is the multiplicative order of 2 (mod k). %C A297362 The corresponding primes are 3, 7, 3, 89, 31, 178481, 5, 7, 3, 23, 11, ... %H A297362 Amiram Eldar, <a href="/A297362/b297362.txt">Table of n, a(n) for n = 1..64</a> %e A297362 5 is in the sequence since ord(2, 5) = 4 and (2^4 - 1)/5 = 3 is prime. %t A297362 aQ[n_] := PrimeQ[(2^MultiplicativeOrder[2, n] - 1)/n]; Select[Range[10000],aQ] %o A297362 (PARI) is(n) = n%2 && isprime((2^znorder(Mod(2, n))-1)/n); \\ _Amiram Eldar_, Aug 26 2023 %Y A297362 Cf. A002326, A165781. %K A297362 nonn %O A297362 1,1 %A A297362 _Amiram Eldar_, Dec 29 2017