Signed magnification sums for general spherical lenses
  It is well known that the sum of signed magnifications is invariant for mass
lens systems. In this paper, we discuss the signed magnification sums of
general spherical lens models including the singular isothermal sphere, the
Schwarzschild lens and the Ellis wormhole, the last of which is an example of
the traversable wormholes of the Morris-Thorne class. We show that the signed
magnication sums are a very useful tool to distinguish exotic lens objects. For
example, we show that one can distinguish the Ellis wormholes from the
Schwarzschild lens with the signed magnification sums.
