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 A367649 #13 Jan 29 2025 07:10:14 %S A367649 7,19,37,163,487,1297,1459,2917,19441,19927,39367,59779,131221,208657, %T A367649 224209,572023,2051893,5062663,8503057,19131877,44457337,86093443, %U A367649 113863969,133923133,258280327,565571323,600830137,859270843,1319934691,4161183031,5366491219,5879415781 %N A367649 Primes p such that the multiplicative order of 3 modulo p is 2 times a power of 3. %C A367649 Odd prime factors of numbers of the form 3^3^i + 1: for odd primes p, p divides 3^3^i + 1 if and only if the multiplicative order of 3 modulo p is 2 times a power of 3 not exceeding 3^i. %e A367649 37 is a term since the multiplicative order of 3 modulo 37 is 18 = 2*3^2, which means that 37 is a factor of 3^3^2 + 1. %e A367649 163 is a term since the multiplicative order of 3 modulo 163 is 162 = 2*3^4, which means that 163 is a factor of 3^3^4 + 1. %o A367649 (PARI) isA367649(n) = my(d); isprime(n) && (n!=3) && ((d=znorder(Mod(3,n)))%2==0) && isprimepower(3*d/2) %Y A367649 Subsequence of A367266. %Y A367649 Cf. A023394 (ord(2,p) being a power of 2, prime factors of numbers of the form 2^2^i - 1 (or of the form 2^2^i + 1)), A367648 (ord(3,p) being a power of 3, prime factors of numbers of the form 3^3^i - 1). %K A367649 nonn %O A367649 1,1 %A A367649 _Jianing Song_, Nov 25 2023 %E A367649 a(28)-a(31) from _Chai Wah Wu_, Nov 26 2023 %E A367649 a(32) from _Jinyuan Wang_, Jan 29 2025