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 A281624 #20 Feb 02 2025 12:10:39 %S A281624 1,2,3,4,5,6,8,10,12,15,16,17,20,24,30,32,34,40,48,60 %N A281624 Numbers m such that 2^phi(m) + 1 is prime (Fermat prime). %C A281624 Numbers m such that A243305(m) is a Fermat prime (A019434). %C A281624 If there are only 5 Fermat primes, sequence is finite with 20 terms; corresponding values of Fermat primes: 3, 3, 5, 5, 17, 5, 17, 17, 17, 257, 257, 65537, 257, 257, 257, 65537, 65537, 65537, 65537, 65537. %C A281624 Number of numbers k such that 2^phi(k) + 1 = A019434(n) for n = 1-5: 2, 3, 4, 5, 6. %e A281624 10 is a term because 2^phi(10) + 1 = 2^4 + 1 = 17 (prime). %t A281624 Select[Range[60], PrimeQ[2^EulerPhi[#] + 1] &] (* _Paolo Xausa_, Jan 18 2025 *) %o A281624 (Magma) [n: n in[1..10000] | IsPrime(2^(EulerPhi(n)) + 1)]; %o A281624 (PARI) is(n)=isprime(2^eulerphi(n)+1) \\ _Charles R Greathouse IV_, Jan 27 2017 %Y A281624 Subsequence of A003401. %Y A281624 Cf. A000010 (phi(n)), A019434, A243305, A281623. %K A281624 nonn,more %O A281624 1,2 %A A281624 _Jaroslav Krizek_, Jan 25 2017