A336719 - OEIS

%I #12 Aug 25 2020 06:40:00

%S 3,7,7,31,31,127,127,127,127,127,127,8191,8191,8191,8191,131071,

%T 131071,524287,524287,524287,524287,524287,524287,524287,524287,

%U 524287,524287,524287,524287,2147483647,2147483647,2147483647,2147483647,2147483647,2147483647

%N Largest odd prime p for which the order of 2 mod p is at most n.

%C a(1) is undefined.

%C Changing "at most n" to "equal to n" in the definition gives A097406.

%C The first term that is not a Mersenne prime (A000668) is 4432676798593.

%C For a version without duplicates, see A336720. For a list of all n where a(n) increases, see A336721.

%H Jeppe Stig Nielsen, <a href="/A336719/b336719.txt">Table of n, a(n) for n = 2..251</a>

%o (PARI) re=0;for(n=2,+oo,p=vecmax(factor(2^n-1)[,1]);p>re&&re=p;print1(re,", "))

%Y Cf. A000668, A005420, A014664, A097406, A336720, A336721.

%K nonn

%O 2,1

%A _Jeppe Stig Nielsen_, Aug 01 2020