%I #20 Aug 24 2024 05:50:56
%S 4,8,20,28,48,60,88,140,160,228,280,308,368,468,580,620,748,840,888,
%T 1040,1148,1320,1568,1700,1768,1908,1980,2128,2688,2860,3128,3220,
%U 3700,3800,4108,4428,4648,4988,5340,5460,6080,6208,6468,6600,7420
%N a(n) = (prime(n)-1)*(prime(n)+1)/6.
%H Vincenzo Librandi, <a href="/A084922/b084922.txt">Table of n, a(n) for n = 3..840</a>
%F a(n) = A084920(n)/6.
%F a(n) = A084921(n)/3.
%t Select[Range[0, 7000], PrimeQ[Sqrt[6 # + 1]]&] (* _Vincenzo Librandi_, Apr 11 2013 *)
%t (Prime[Range[3,60]]^2 -1)/6 (* _G. C. Greubel_, May 02 2024 *)
%o (Magma) [(p^2-1)/6: p in PrimesInInterval(4, 250)]; // _Vincenzo Librandi_, Apr 11 2013
%o (PARI) a(n) = (prime(n)^2-1)/6; \\ _Michel Marcus_, Mar 22 2016
%o (SageMath) [(n^2-1)//6 for n in prime_range(4,301)] # _G. C. Greubel_, May 02 2024
%Y Cf. A000040, A006093, A008864, A084920, A084921.
%K nonn,easy
%O 3,1
%A _Reinhard Zumkeller_, Jun 11 2003