Carry out an elliptic integral:
Definition of the solid angle subtended by a disk (for instance a detector, a road sign) at the origin in the
x,
y-plane from a point

:
Closed form result for the solid angle:
Plot the solid angle as a function of horizontal distance and height:
This calculates the classical action for a relativistic 3D oscillator:
The action can be expressed using
EllipticPi (for brevity, occurring roots are abbreviated):
Visualize the image of lines of constant real and imaginary parts: