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How to prove that a power series in more than one variable is invertible?

Is this still true if the variables are noncommutative?

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    @Stultus: if by "invertible" you mean "units in the power series ring", then not every power series in more than one variable is invertible. For example, $(xy) + (xy)^2 + \cdots + (xy)^n + \cdots$ is not invertible in $\mathbb{R}[[x,y]]$.2010-11-22

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