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Smallest nonnegative value taken on by n*x^2 - y^2 for an infinite number of integer pairs (x, y).
1

%I #9 Oct 11 2016 03:26:02

%S 0,1,2,0,1,2,3,4,0,1,2,3,1,5,6,0,1,2,2,4,3,2,7,8,0,1,2,3,1,5,3,7,8,9,

%T 10,0,1,2,3,4,1,6,2,7,5,5,11,12,0,1,2,3,1,2,6,7,3,1,2,11,1,13,14,0,1,

%U 2,2,4,5,5,7,8,1,1,11,3,7,14,15,16,0,1,2,3,1,2,6,7,1,9,3,7,3,5,14,15,1

%N Smallest nonnegative value taken on by n*x^2 - y^2 for an infinite number of integer pairs (x, y).

%t f[n_, z_] := FindInstance[x > 0 && y > 0 && n*x^2 - y^2 == z, {x, y}, Integers, 1]; a[n_] := For[z = 0, True, z++, fz = f[n, z]; If[fz =!= {}, Print["a(", n, ") = ", z, " {x, y} = ", {x, y} /. fz[[1]]]; Return[z]]]; Array[a, 97] (* _Jean-François Alcover_, Oct 11 2016 *)

%K nonn

%O 1,3

%A _David W. Wilson_