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Let $k$ be a field and let $X_1, X_2, \ldots , X_n$ be formal noncommuting variables and let $K\langle \langle X_1, X_2, \ldots , X_n\rangle \rangle $ be the formal noncommutative power series ring in these variables.

Suppose $F$ is a power series in this ring such that its constant term is nonzero. How to prove that $F^{-1}$ exists?

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    $k=K$?${}{}{}{}$2011-11-29

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