A257411 Values of n such that there are exactly 4 solutions to x^2 - y^2 = n with x > y >= 0.

96, 105, 120, 135, 160, 165, 168, 189, 195, 216, 224, 231, 255, 256, 264, 273, 280, 285, 297, 312, 345, 351, 352, 357, 375, 385, 399, 408, 416, 420, 429, 435, 440, 455, 456, 459, 465, 483, 512, 513, 520, 540, 544, 552, 555, 561, 595, 608, 609, 615, 616, 621

Offset: 1

Author

Colin Barker, Apr 22 2015

Keywords

• nonn

Examples

			96 is in the sequence because there are 4 solutions to x^2 - y^2 = 96, namely (x,y) = (10,2), (11,5), (14,10), (25,23).
		

Links

• Colin Barker, Table of n, a(n) for n = 1..600

Crossrefs

Cf. A257408, A257409, A257410, A257412, A257413, A257414, A257415, A257416, A257417.

Cf. A034178.

Programs

Mathematica

nn = 1000;
t = Table[0, {nn}];
Do[n = x^2 - y^2; If[n <= nn, t[[n]]++], {x, nn}, {y, 0, x - 1}];
Position[t, 4] // Flatten (* Jean-François Alcover, Jun 18 2020, after T. D. Noe in A034178 *)

PARI

is_A257411(n)={A034178(n)==4} \\ M. F. Hasler, Apr 22 2015