How is $\mathbb C(x)^*$ a free group?

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My professor gave an example of a free group as the field of rationals over one variable $\mathbb C(x)^*$ but I do not know how. could anyone explain this to me, please?

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You mean a free abelian group, a free group is completely different.

$\Bbb{C} (x)^\times/\Bbb{C}^\times$ (isomorphic to the quotients of monic polynomials) is a free abelian group, generated by the $x-a,a\in \Bbb{C}$.

$\Bbb{C} (x)^\times$ is not a free abelian group as $(-1)^2= 1$. Also for any fixed $z\in \Bbb{C}^\times$, there is a $z^{1/n}$ for all $n$.