  Bound states around an impurity are investigated for a two dimensional
electron system in a strong magnetic field. Long-range Coulomb potential and
related potentials are considered. Schr\"odinger equation is solved numerically
to obtain the bound states. The energy and wave function of these bound states
are indirectly observed by the scanning tunneling spectroscopy as local density
of states (LDOS). Theoretically obtained LDOS is compared with experiment.
Reasonable agreement is obtained.
