%N 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 Norms of numbers in O_Q(sqrt(-7)).
%C A033207 and A045386 are subsets of this sequence. - _Colin Barker_, Sep 29 2014
%e 1/4 + 7/4 = 2, so 2 is in the sequence. (This also means 2 is composite in O_Q(sqrt(-7))).
%e 2^2 + 7 * 0^2 = 4, so 4 is in the sequence.
%e 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 Cf. A002481, A033207, A045386.
%A _Alonso del Arte_, Sep 28 2014