Which of the following equations represents a Pell equation?
x2+y2=1x^2 + y^2 = 1x2+y2=1
x2−2y2=1x^2 - 2y^2 = 1x2−2y2=1
x+y=1x + y = 1x+y=1
x2−y2=0x^2 - y^2 = 0x2−y2=0