This expression gives an apparently different answer for the case of p 2 q 2 from the totally symmetric expression above. There is no contradiction, however, since the canonical commutation relations allow for more than one expression for the same operator. (The reader may find it instructive to use the commutation relations to rewrite the totally symmetric formula for the case of p 2 q 2 in terms of the operators P 2 Q 2 , Q P 2 Q , and Q 2 P 2 and verify the first expression in McCoy's formula with m = n = 2 .)