A280004 - OEIS

A280004

Numbers k such that 9*2^k + 1 is a prime factor of a Fermat number 2^(2^m) + 1 for some m.

%I #59 Feb 16 2025 08:33:38

%S 67,9431,461081,2543551

%N Numbers k such that 9*2^k + 1 is a prime factor of a Fermat number 2^(2^m) + 1 for some m.

%C Fernando (Remark 5.2) shows that all terms are odd. - _Jeppe Stig Nielsen_, Jan 02 2025

%H R. Fernando, <a href="https://ravif.web.illinois.edu/fermat_divisors.pdf">Morehead-like restrictions on Fermat divisors</a>.

%H Proth Search Page, <a href="http://www.prothsearch.com/fermat.html">Prime factors of Fermat numbers</a>

%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatNumber.html">Fermat Number</a>

%Y Subsequence of A002256.

%Y Cf. A000215, A050528, A057778, A201364, A204620, A226366, A280003.

%K nonn,hard,more,changed

%O 1,1

%A _Arkadiusz Wesolowski_, Feb 21 2017