http://mathworld.i2p/DeltaFunction.html
The delta function can also be defined by the limit as (41) Delta functions can also be defined in two dimensions, so that in two-dimensional Cartesian coordinates (42) (43) (44) and (45) Similarly, in polar coordinates , (46) (Bracewell 1999, p. 85). In three-dimensional Cartesian coordinates (47) (48) and (49) in cylindrical coordinates , (50) In spherical coordinates , (51) (Bracewell 1999, p. 85).