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
Keywords
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
Programs
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Mathematica
Select[Range@510, PrimeNu[#^2 + 1] == 4 &] (* Robert G. Wilson v, Jul 15 2018 *)
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PARI
isok(n) = omega(n^2+1) == 4; \\ Michel Marcus, Jun 30 2018