A038844 Numbers k for which 6*k+1 divides 2^k-1.

5, 21, 37, 72, 73, 76, 100, 121, 153, 221, 233, 237, 245, 276, 288, 292, 296, 300, 305, 333, 336, 341, 348, 352, 357, 380, 381, 397, 445, 448, 461, 465, 472, 492, 545, 557, 565, 576, 577, 601, 605, 637, 648, 657, 676, 688, 692, 696, 737, 752, 753, 761, 776

Offset: 1

Author

Bill Gosper

Keywords

• nonn

Comments

Apart from 5, all terms are in A045762, numbers k such that 2^k-1 is not prime. - Michel Marcus, Nov 12 2014

6*k + 1 is not necessarily a prime for k being a term of this sequence. - Jianing Song, Jun 20 2025

Examples

			For n=5, 2^5-1=31 is divisible by 6*5+1=31.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A045762, A188130 (subsequence of primes).

Programs

Mathematica

Select[Range[800],PowerMod[2,#,6#+1]==1&] (* Harvey P. Dale, Oct 24 2017 *)

PARI

select( {is_A038844(n)=Mod(2,n*6+1)^n==1}, [1..999]) \\ M. F. Hasler, Aug 17 2021

Extensions

More terms from Michel Marcus, Nov 12 2014