A182091 Numbers n such that (n-1)*n^2/2-1 is prime.

4, 6, 8, 10, 13, 17, 18, 21, 22, 25, 30, 34, 36, 38, 41, 42, 45, 46, 56, 57, 58, 62, 64, 69, 70, 72, 74, 86, 88, 101, 105, 113, 114, 116, 120, 122, 126, 132, 133, 141, 157, 158, 174, 176, 181, 182, 186, 192, 198, 200, 205, 216, 217, 221, 224, 240, 253, 254

Offset: 1

Author

Gerasimov Sergey, Apr 11 2012

Keywords

• nonn
• easy

Examples

			a(1)=4 because (4-1)*4^2/2-1=23 is prime, a(2)=6 because (6-1)*6^2/2-1=89 is prime.
		

Links

• G. C. Greubel, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. A006002.

Programs

Magma

[n: n in [1..300] |IsPrime((n-1)*n^2 div 2-1)]; // Vincenzo Librandi, Aug 21 2017

Mathematica

Select[Range[300], PrimeQ[(# - 1)*#^2/2 - 1] &] (* T. D. Noe, Apr 11 2012 *)

PARI

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