cp's OEIS Frontend

A217755 Numbers n such that ((n^2 + n)/2)^2 + 1 is prime.

%I A217755 #33 Sep 08 2022 08:46:04
%S A217755 1,3,4,8,11,15,20,24,28,31,39,43,48,64,75,76,79,80,111,116,120,135,
%T A217755 148,155,160,168,179,184,200,203,208,211,216,223,224,235,243,251,263,
%U A217755 264,288,303,319,320,324,348,351,356,364,371,375,388,416,419,420,424,428,439,443,451,456,459
%N A217755 Numbers n such that ((n^2 + n)/2)^2 + 1 is prime.
%C A217755 Also, numbers n such that the sum of the first n cubes precedes a prime.
%e A217755 1 is in the sequence because ((1^2 + 1)/2)^2 + 1 = 2 and 2 is prime;
%e A217755 3 is in the sequence because ((3^2 + 3)/2)^2 + 1 = 37 and 37 is prime.
%t A217755 Select[Range[400], PrimeQ[((#^2 + #)/2)^2 + 1] &] (* _Alonso del Arte_, Mar 26 2013 *)
%o A217755 (PARI) for(n=1,10^3, if(isprime(((n^2 + n)/2)^2 + 1), print1(n,", "))); /* _Joerg Arndt_, Mar 27 2013 */
%o A217755 (Magma) [n: n in [1..500] | IsPrime(s+1) where s is (n^2+n)^2 div 4]; // _Bruno Berselli_, Mar 27 2013
%Y A217755 Cf. A067186.
%K A217755 nonn
%O A217755 1,2
%A A217755 _Luca Brigada Villa_, Mar 26 2013