The function reversion(a, b) returns the formal power series that is the reversion of a with respect to b.  If b is not specified then it is assumed to be the formal power series with one nonzero coefficient, .

Since reversion is the inverse of composition, composition of the result into a will give b.

Note that  must be ,  must be , and  must be . If not, the reversion is not well defined and reversion returns an error message.

The command with(powseries,reversion) allows the use of the abbreviated form of this command.