The Reciprocal of $\sum_{n\geq 0}a^nb^n$ for non-commuting $a$ and $b$,   Catalan numbers and non-commutative quadratic equations
  The aim of this paper is to describe the inversion of the sum $\sum_{n\geq 0}a^nb^n$ where $a$ and $b$ are non-commuting variables as a formal series in $a$ and $b$. We show that the inversion satisfies a non-commutative quadratic equation and that the number of certain monomials in its homogeneous components equals to a Catalan number. We also study general solutions of similar quadratic equations. 
Arkady Berenstein, Vladimir Retakh, Christophe Reutenauer and Doron   Zeilberger
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