 Solution

The automorphisms satisfy ${\sigma }^n={\tau }^2=1$ and $\tau \sigma \tau ={\sigma }^{-1}$. The $n$ rotations $z\mapsto {\zeta }_n^{j}z$ and $n$ inversion maps $z\mapsto {\zeta }_n^j/z$ are all distinct, so the dihedral group $G$ has order $2n$, including the case $n=1$ when it has order two. The invariantis fixed by both generators.

Use two standard field-theoretic results: the fixed-field theorem for a finite group of automorphisms gives $[L:L^G]=|G|$ and makes $L/L^G$ Galois with group $G$; and a nonconstant rational function $P(z)/Q(z)$ in lowest terms satisfies $[\mathbb{C}(z):\mathbb{C}(P/Q)]=\max (\mathrm{deg}P,\mathrm{deg}Q)$. Here $w=(z^{2n}+1)/z^n$ has coprime numerator and denominator and degree $2n$. Thus $[L:\mathbb{C}(w)]=[L:L^G]=2n$. Since $\mathbb{C}(w)\subseteq L^G$, the tower law proves the dihedral fixed field in one rational variable: