%N Values of n for which the equation x^2 - 25*y^2 = n has integer solutions.
%C This equation is a Pellian equation of the form x^2 - D^2*y^2 = N.
%H Colin Barker, <a href="/A239435/b239435.txt">Table of n, a(n) for n = 1..1000</a>
%e 21 is in the sequence because the equation x^2 - 25*y^2 = 21 has the solution (X,Y) = (11,2).
%o b(n) = sumdiv(n, f, f^2<=n && (n-f^2)%(10*f)==0) \\ See A239434
%o s=; for(n=0, 250, if(b(n)>0, s=concat(s,n))); s
%Y Cf. A042965, A230239, A230240, A239434.
%A _Colin Barker_, Mar 18 2014