This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247979 #12 Oct 03 2014 17:58:58 %S A247979 0,1,2,4,7,8,9,11,14,16,18,21,22,23,25,28,29,32,36,37,43,44,46,49,50, %T A247979 53,56,58,63,64,67,71,72,74,77,79,81,86,88,92,98,99,100,106,107,109, %U A247979 112,113,116,121,126,127,128,134,137,142,144,148,149,151,154,158,161,162,163,169,172 %N A247979 Numbers of the form x^2 + 7y^2 with x, y integers, or x^2/4 + 7y^2/4 with x, y odd integers. %C A247979 Norms of numbers in O_Q(sqrt(-7)). %C A247979 A033207 and A045386 are subsets of this sequence. - _Colin Barker_, Sep 29 2014 %e A247979 1/4 + 7/4 = 2, so 2 is in the sequence. (This also means 2 is composite in O_Q(sqrt(-7))). %e A247979 2^2 + 7 * 0^2 = 4, so 4 is in the sequence. %e A247979 There is no way to express 5 as x^2 + 7y^2, nor as x^2/4 + 7y^2/4 if x and y are constrained to odd integers, hence 5 is not in the sequence. (This also means 5 is prime in O_Q(sqrt(-7)) and its norm is 25). %Y A247979 Cf. A002481, A033207, A045386. %K A247979 easy,nonn %O A247979 1,3 %A A247979 _Alonso del Arte_, Sep 28 2014