[QUE/CM-05013]

  For each of the potentials below check if a circular orbit is possible or not.

1.       $\displaystyle{ -V_0 {\exp(-\alpha r)\over r}}$ 
2.       $\displaystyle -{ \alpha \over r^4}$
3.       $\displaystyle -{\alpha \over r^2}$

  If the orbit exists, relate the radius of the orbit to the angular  momentum and energy.