 formula_12 is called an "annihilating operator" of "c" (the annihilating operators of formula_25 form an ideal in the ring formula_26, called the "annihilator" of formula_25).The quantity "r" is called the "order" of the annihilating operator.By extension, the holonomic sequence "c" is said to be of order "r" when an annihilating operator of such order exists.