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A316351 Numbers k such that k^2 + 1 has exactly four distinct prime factors. 1
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 (list; graph; refs; listen; history; text; internal format)
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 A052231 A092178

Adjacent sequences:  A316348 A316349 A316350 * A316352 A316353 A316354

KEYWORD

nonn

AUTHOR

Gordon Elliot Michaels, Jun 29 2018

STATUS

approved

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Last modified February 19 18:30 EST 2019. Contains 320327 sequences. (Running on oeis4.)