A367002 - OEIS

%I #25 Oct 29 2024 03:29:44

%S 7,23,3,3,383,5,23,17,3,3,23,5,5,7,3,3,79,13,1879,13,3,3,47,7,229,5,3,

%T 3,32212254719,263,223,5,3,3,5,73,17,1217,3,3,6709,29,7,71,3,3,11,97,

%U 47,228713,3,3,5,37,5,7,3,3,9377,11,13,479,3,3,41,5,13,137

%N a(n) is the smallest prime factor of n*2^n-1.

%H Amiram Eldar, <a href="/A367002/b367002.txt">Table of n, a(n) for n = 2..884</a> (terms 2..325 from Robert Israel)

%F a(n) = A020639(A003261(n)).

%F a(n) = 3 iff n == 4 or 5 (mod 6). - _Robert Israel_, Nov 08 2023

%p f:= n -> min(numtheory:-factorset(n*2^n-1)):

%p map(f, [$2..100]); # _Robert Israel_, Nov 08 2023

%t Table[FactorInteger[n*2^n-1][[1,1]], {n,2,69}] (* _Paul F. Marrero Romero_, Dec 17 2023 *)

%Y Cf. A003261, A020639, A049479, A367003, A367004, A366899.

%K nonn

%O 2,1

%A _Sean A. Irvine_, Oct 31 2023