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

Author

Dimitris Valianatos, Apr 24 2021

Keywords

• nonn

Examples

			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.
		

Crossrefs

Cf. A071904, A129820.

Programs

PARI

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

Formula

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