A187371 Least odd number k such that (k*2^n-1)*k*2^n-1 or (k*2^n-1)*k*2^n+1 or (k*2^n+1)*k*2^n-1 or (k*2^n+1)*k*2^n+1 is prime.

1, 1, 1, 1, 1, 1, 3, 3, 1, 1, 5, 5, 3, 9, 7, 1, 5, 1, 3, 19, 9, 11, 7, 1, 11, 1, 27, 25, 9, 11, 3, 1, 21, 19, 1, 19, 3, 1, 7, 9, 9, 3, 27, 5, 1, 5, 5, 7, 3, 1, 41, 1, 9, 17, 1, 11, 1, 27, 3, 105, 33, 15, 23, 29, 19, 43, 37, 15, 3, 11, 27, 9, 57, 5, 3, 3, 35

Offset

1,7

Comments

As N increases, it appears that (sum_{k=1..N} a(k)) / (sum_{k=1..N} k) tends to 0.24

Links

Pierre CAMI, Table of n, a(n) for n = 1..4000

Formula

a(n) = least of A187367(n), A187368(n), A187369(n), A187370(n).

Mathematica

Table[k = 1; While[! PrimeQ[(k*2^n - 1)*k*2^n - 1] && ! PrimeQ[(k*2^n - 1)*k*2^n + 1] && ! PrimeQ[(k*2^n + 1)*k*2^n - 1] && ! PrimeQ[(k*2^n + 1)*k*2^n + 1], k = k + 2]; k, {n, 100}]

Crossrefs

Cf. A187367, A187368, A187369, A187370.

Sequence in context: A046218 A046221 A220837 * A056611 A177407 A135669

Adjacent sequences: A187368 A187369 A187370 * A187372 A187373 A187374

Keyword

nonn

Author

Pierre CAMI, Mar 09 2011

Status

approved