This is a rearrangement of the equation

x^2 + (F(x))^2 = a^2

which is the equation for a circle of radius

The picture is a plot for a=1.

x^2 + (F(x))^2 = a^2

which is the equation for a circle of radius

**a**centered at the origin. If only the positive root is taken, then you get only the upper half of the circle.The picture is a plot for a=1.