Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 3 / 3 / b

Put $u=t-p$ and write $a(t)=u^{r}b(u)$, where $b(0)\ne 0$. The unit $b$ has a square root in $\mathbb{C}[[u]]$. Dividing the radical by that square root and by $u^{\lfloor r/2\rfloor }$ reduces the completed extension toIf $r$ is even, it is unramified and locally splits; if $r$ is odd, adjoining $u^{1/2}$ gives ramification index two. At points where $a(p)\ne 0$, $r=0$ and there is no branch. ThereforeFor infinity set $u=1/t$. If $D=\mathrm{deg}a$,The same quadratic branch parity criterion applies to the negative valuation $-D$, soThe generic irreducibility excludes $a$ being a square. Local splitting at the even-order places is compatible with that global irreducibility. The number of odd finite orders plus the possible odd infinity order is even, as also follows from the sum of orders of a rational function being zero.