A316351 Numbers k such that k^2 + 1 has exactly four distinct prime factors.

47, 73, 83, 123, 133, 157, 173, 177, 183, 187, 191, 203, 213, 217, 233, 237, 242, 253, 255, 265, 273, 278, 293, 302, 307, 313, 317, 319, 327, 333, 337, 343, 353, 377, 387, 395, 401, 403, 411, 413, 421, 423, 437, 438, 467, 473, 477, 483, 487, 489, 497, 499, 507

Offset: 1

Author

Gordon Elliot Michaels, Jun 29 2018

Keywords

• nonn

Examples

			For k = 133, k^2 + 1 = 17690 = 2*5*29*61 which has 4 distinct prime factors, so 133 is a term.
For k = 157, k^2 + 1 = 24650 = 2*5*5*17*29 which has 4 distinct prime factors, so 157 is a term.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001221, A002522, A033993.

Programs

Mathematica

Select[Range@510, PrimeNu[#^2 + 1] == 4 &] (* Robert G. Wilson v, Jul 15 2018 *)

PARI

isok(n) = omega(n^2+1) == 4; \\ Michel Marcus, Jun 30 2018