 This can also be written as formula_7 where

and formula_9 is the differential operator that maps formula_10 to formula_11.formula_12 is called an "annihilating operator" of "f" (the annihilating operators of formula_13 form an ideal in the ring formula_14, called the "annihilator" of formula_13).The quantity "r" is called the "order" of the annihilating operator.