A190638 Numbers n such that with b=n*(2n-1) two remainders x and y are defined via x = 2^(b-1) -1 mod b and y = (2*n-1)^(b-1) - 1 mod b which satisfy x==y==0 (mod n) and y-x=n.

5, 41, 257, 2309, 14621, 48821, 125429, 177269, 1595417, 5278001, 10596137, 15146069, 21523361, 63993929, 83629517, 111321257, 363526421, 375805589, 427518041, 446072909, 552010829, 752665649, 1980098177

Offset

1,1

Comments

The first 23 terms (at least) are primes.

Conjecture: All terms have the form 12*k+5.

The first composite numbers in the sequence are 2^80+1 and 2^512+1.

If we modify one of the conditions to y - x = 2*n, the sequence changes to 3, 7, 19, 31, 79, 139, 199, 211, 271, 283, 307, 331, 367, 379, 439, 499, 511, ...

or if we modify it to y - x = 64*n, the sequence becomes 89, 101, 197, 269, 317, 341, 461, 521, 569, 701, 821, 857, 881, 929, 1109, 1181, 1217, ...

There seem to be no solutions n if the condition is modified to any y - x <= 0.

Links

Table of n, a(n) for n=1..23.

Example

For n=41, b = 41*(2*41-1)=3321. So 2^3320 == 3199 (mod 3321) leads to x = 3199 - 1 = 3198 which satisfies x == 0 (mod 41), and 81^3320 == 3240 (mod 3321) leads to y = 3240 - 1 = 3239 which satisfies y == 0 (mod 41) and y - x = 41. Therefore n=41 is in the sequence.

Maple

isA190638 := proc(n) local b, x, y; b := n*(2*n-1) ; x := modp( 2 &^ (b-1), b) -1; y := modp( (2*n-1) &^ (b-1), b) -1; if y-x =n and modp(x, n) = 0 and modp(y, n) = 0 then true; else false; end if; end proc:
for n from 2 do if isA190638(n) then print(n); end if; end do: # R. J. Mathar, Jun 04 2011

Crossrefs

Sequence in context: A273313 A356198 A198725 * A083300 A308247 A223009

Adjacent sequences: A190635 A190636 A190637 * A190639 A190640 A190641

Keyword

nonn

Author

Alzhekeyev Ascar M, May 15 2011

Status

approved