A138218 Numbers k such that 180k^2 + 1 is prime.

1, 3, 6, 7, 14, 17, 18, 22, 24, 25, 27, 28, 29, 31, 32, 41, 46, 48, 50, 52, 55, 59, 62, 64, 66, 67, 76, 77, 83, 85, 87, 88, 92, 94, 95, 97, 102, 106, 108, 118, 123, 134, 136, 139, 140, 141, 147, 148, 154, 155, 157, 162, 165, 167, 179, 181, 192, 193, 199, 202, 203, 207

Offset: 1

Author

Zak Seidov, May 05 2008

Keywords

• nonn
• easy

Comments

Or, numbers j arising in A137530.

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A137530.

Programs

Magma

[n: n in [0..10000]|IsPrime(1+180*n^2)] // Vincenzo Librandi, Dec 13 2010

Mathematica

Select[Range[250],PrimeQ[180#^2+1]&] (* Harvey P. Dale, May 26 2024 *)

PARI

is(n)=isprime(1+180*n^2) \\ Charles R Greathouse IV, Jun 13 2017

Formula

a(n) = sqrt((A137530(n) - 1)/180).