Recent content by Ted Jacobson

A geometry of two dimensions, like the surface of a sphere or an egg, has a single number that characterizes it's intrinsic curvature at each point. The "radius of curvature" is defined as the inverse of the square root of the curvature. In three or higher dimensions, to characterize the...

hi

The pnd’s are the directions in which a geodesic congruence of light rays experiences no shear, i.e. no astigmatism.