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 A089920 #29 Aug 28 2025 07:08:53 %S A089920 1,4,11,149 %N A089920 Indices of primes p such that 7^p - 2 is prime. %C A089920 Except for p=2, 2, 5^p - 2 cannot be prime. This immediately follows from the fact that a number N = (3k+2)^p - 2 cannot be prime for p > 2 because N = 3H + 2^p - 2 = 3H + 2(2^(p-1)-1) is divisible by 3. %C A089920 a(5) > 8742, if it exists (cf. _J.W.L. (Jan) Eerland_'s comment in A147782). - _Amiram Eldar_, Jul 07 2024 %C A089920 a(5) > 23967, if it exists (using A090669). - _Michael S. Branicky_, Aug 17 2024 %F A089920 a(n) = A000720(A147782(n)). - _Amiram Eldar_, Jul 07 2024 %t A089920 Select[Range[500],PrimeQ[7^Prime[#]-2]&] (* _Harvey P. Dale_, May 02 2011 *) %t A089920 Position[Prime[Range[150]],_?(PrimeQ[7^#-2]&)]//Flatten (* _Harvey P. Dale_, May 11 2016 *) %o A089920 (PARI) forprime(p=2,1e4,if(ispseudoprime(7^p-2),print1(primepi(p),","))) %Y A089920 Cf. A147782 (primes p such that 7^p - 2 is prime), A000720, A090669. %K A089920 nonn,more,hard,changed %O A089920 1,2 %A A089920 _Cino Hilliard_, Jan 11 2004 %E A089920 Definition clarified by _Harvey P. Dale_, May 02 2011