1,2

This gives a much sparser sequence than the sequence A006521 of all solutions to the equation n | 2^n+1. An element of A006521 is said to be *primitive* if it is not divisible by any smaller element of A006521 having the same prime divisors, and further it is not the lcm of any two smaller elements of A006521. See Proposition 1 of the link.

Every element of this sequence apart from 1 and 3 is divisible either by 27 or by 171. Stronger results hold. For instance, every element of this sequence apart from 1 and 3 is divisible either by 171 or 243 or 13203 or 2354697 or 10970073 or 22032887841. See the link or A136475 for more details about such results. These alternative factors enable the sequence to be generated much more quickly than by the short Maple program given below.

Toby Bailey and Chris Smyth, Primitive solutions of n|2^n+1.

9 is in A006521 but is not primitive because its set of prime divisors is the same as that of 3, which divides 9 and is in A006521.

250857 is in A006521 but not primitive, as 250857=lcm(171,13203), and both 171 and 13203 are in A006521.

L:=1: S:={}: for j from 3 by 6 to 10^7 do if not 2&^j+1 mod j = 0 then next end if; if not (j in S) then L := L, j end if; S := S union map( ilcm, S, j ) union {j}; S := S union map(`*`, {map2( op, 1, ifactors(j)[2] )[]}, j); end do: L;

Cf. A006521, A136474, A136475.

Adjacent sequences: A136470 A136471 A136472 this_sequence A136474 A136475 A136476

Sequence in context: A116506 A032484 A119117 this_sequence A053930 A053920 A125711

easy,nonn

Toby Bailey and Chris Smyth (c.smyth(AT)ed.ac.uk), Jan 13 2008