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 A334993 #18 Feb 11 2021 10:19:40 %S A334993 1,5,9,17,57,65,897,4217,6225,152529,3648969,5570081 %N A334993 Numbers k such that 2*3^k + 1 is prime and divides Phi(3^m, 2). %C A334993 A subset of odd values from A003306. %C A334993 If p = 2*3^k + 1 is prime then p divides 2^(3^k) + (-1)^k, due to Euler's criterion. %C A334993 Only odd terms of sequence A003306 can divide the cyclotomic expression Phi(3^m, 2); none of the even terms of sequence A003306 can divide 2^3^k-1 and therefore cannot divide Phi(3^m, 2). %H A334993 C. Caldwell's <a href="https://primes.utm.edu/top20/page.php?id=37">Prime Page for Divides Phi category</a>. %o A334993 (PARI) dp(n)=Mod(2,2*3^n+1)^3^n==1; %o A334993 forstep(n=1,6225,2,if(dp(n),print1(n,", "))) %Y A334993 Cf. A003306. %K A334993 nonn,hard,more %O A334993 1,2 %A A334993 _Serge Batalov_, May 18 2020