A173288 - OEIS

%I #5 Apr 26 2013 18:03:48

%S 2,3,4,5,6,8,12,13,14,15,16,17,19,20,21,24,25,27,28,33,38,41,43,49,50,

%T 54,55,57,59,62,66,69,71,75,76,77,78,79,80,88,89,90,99,101,105,106,

%U 110,111,117,118,119,121,131,138,139,141,143,145,147,150,151,153,154,155

%N Either n^2+n+{1,9} is prime.

%C Or numbers n such that either n-th oblong number plus 1 or 9 is prime.

%C Numbers n such that n^2+n+1 or n^2+n+9 is prime, but not both. [From _R. J. Mathar_, Feb 21 2010]

%e a(1)=1 because 1^2+1+1=4=nonprime and 1^2+1+9=11=prime; a(2)=2 because 2^2+2+1=7=prime and 2^2+2+9=15=nonprime; a(3)=3 because 3^2+3+1=13=prime and 3^3+3+9=21=nonprime.

%t Select[Range[200],Xor@@PrimeQ[#^2+#+{1,9}]&] (* _Harvey P. Dale_, Apr 26 2013 *)

%Y Cf. A000040, A002378.

%K nonn

%O 1,1

%A _Juri-Stepan Gerasimov_, Feb 15 2010

%E 1 and 11 removed by _R. J. Mathar_, Feb 21 2010