A339480 Numbers of the form (k^2 - 2) / 2 where k - 1 and k + 1 are both odd composite numbers.

337, 577, 1249, 1567, 2047, 2887, 3697, 4231, 4417, 6727, 6961, 7199, 7441, 7687, 8977, 10081, 10367, 10657, 11857, 12799, 14449, 15487, 16927, 17297, 17671, 20401, 20807, 21217, 21631, 22897, 23327, 23761, 24199, 27847, 29767, 30257, 30751, 32257, 33799, 35377, 37537, 40897

Offset

1,1

Links

Table of n, a(n) for n=1..42.

Formula

a(n) = (A129820(2*n - 1) * A129820(2*n) - 1) / 2.

Example

For k = 26, k - 1 = 25 and k + 1 = 27 are both odd composite numbers. So (26^2 - 2) / 2 = 337 is a term of the sequence.

Prog

(PARI) k = 1; forcomposite(c = 1, 287, if(c%2 <> 0, if(c-k == 2, print1((c * (c - 2) - 1) / 2", ")); k = c))

Crossrefs

Cf. A071904, A129820.

Sequence in context: A020358 A260540 A051962 * A214492 A157999 A152853

Adjacent sequences: A339477 A339478 A339479 * A339481 A339482 A339483

Keyword

nonn

Author

Dimitris Valianatos, Apr 24 2021

Status

approved