A084922 a(n) = (prime(n)-1)*(prime(n)+1)/6.

4, 8, 20, 28, 48, 60, 88, 140, 160, 228, 280, 308, 368, 468, 580, 620, 748, 840, 888, 1040, 1148, 1320, 1568, 1700, 1768, 1908, 1980, 2128, 2688, 2860, 3128, 3220, 3700, 3800, 4108, 4428, 4648, 4988, 5340, 5460, 6080, 6208, 6468, 6600, 7420

Offset: 3

Author

Reinhard Zumkeller, Jun 11 2003

Keywords

• nonn
• easy

Links

• Vincenzo Librandi, Table of n, a(n) for n = 3..840

Crossrefs

Cf. A000040, A006093, A008864, A084920, A084921.

Programs

Magma

[(p^2-1)/6: p in PrimesInInterval(4, 250)]; // Vincenzo Librandi, Apr 11 2013

Mathematica

Select[Range[0, 7000], PrimeQ[Sqrt[6 # + 1]]&] (* Vincenzo Librandi, Apr 11 2013 *)
(Prime[Range[3,60]]^2 -1)/6 (* G. C. Greubel, May 02 2024 *)

PARI

a(n) = (prime(n)^2-1)/6; \\ Michel Marcus, Mar 22 2016

SageMath

[(n^2-1)//6 for n in prime_range(4,301)] # G. C. Greubel, May 02 2024

Formula

a(n) = A084920(n)/6.

a(n) = A084921(n)/3.