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

1,1

Links

Table of n, a(n) for n=1..53.

Example

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.

Mathematica

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

Prog

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

Crossrefs

Cf. A001221, A002522, A033993.

Sequence in context: A097458 A094335 A300165 * A282633 A369957 A052231

Adjacent sequences: A316348 A316349 A316350 * A316352 A316353 A316354

Keyword

nonn

Author

Gordon Elliot Michaels, Jun 29 2018

Status

approved