Editing Formal power series (section)

====Operations====
All of the operations defined for series in one variable may be extended to the several variables case.

* A series is invertible if and only if its constant term is invertible in ''R''.
* The composition ''f''(''g''(''X'')) of two series ''f'' and ''g'' is defined if ''f'' is a series in a single indeterminate, and the constant term of ''g'' is zero. For a series ''f'' in several indeterminates a form of "composition" can similarly be defined, with as many separate series in the place of ''g'' as there are indeterminates.

In the case of the formal derivative, there are now separate [[partial derivative]] operators, which differentiate with respect to each of the indeterminates. They all commute with each other.