%N Exceptional even numbers D that do not admit a solution to the Pell equation X^2 - D Y^2 = +2.
%C These are the even numbers D = 2*d with odd d having no prime factors 3 or 5 (mod 8), and do not represent +2 by the indefinite binary quadratic form X^2 - D*Y^2 (with discriminant 4*D > 0).
%C These even D numbers satisfy the necessary condition given in A261246. This condition is not sufficient as the present numbers show.
%C a(n)/2 = d(n) is 7 (mod 8) for n = 24, 31, 48, 55, 57, ...
%C The numbers D which admit solutions to the Pell equation X^2 - D Y^2 = +2 are given by A261246.
%C The exceptional odd D numbers are given in A263010.
%Y Cf. A261246,A263010, A261247, A261248.
%A _Wolfdieter Lang_, Nov 12 2015