See [here][1]. Crossposted from math.stackexchange since there's no good answer despite $>$ 20 upvotes.

> Let $k$ be a field of characteristic $\neq 2$, and consider the quadratic extension $F$ of $k(T)$ generated by $\sqrt{T^3 + 1}$. 

> I'm asking the question: what is the cokernel of $O_S \to F_\infty/O_\infty$ and its dimension?

> The progress I have so far: let $S = \{\infty\} \subset X$ ($X$ is the set of all places of $F$) is the unique place of $F$ that $\text{ord}_\infty(T) < 0$. Note that $O_S = k[T, \sqrt{T^3 + 1}]$. Denote $T$ by $x$ and denote $\sqrt{T^3 + 1}$ by $y$ (so $y^2  =x^3 + 1$). Note also that the integral closure of $k[1/T]$ in $F$ is $k[1/x,\, y/x^2]$ and we have$$\left({y\over{x^2}}\right)^2 = \left({1\over{x}}\right)^4 + {1\over{x}}.$$But I am not sure what to do from here on out.

> EDIT: Maybe Lagrange interpolation might be helpful? 


  [1]: http://math.stackexchange.com/questions/1290075/meromorphic-functions-on-u2-t3-1-cokernel-of-o-s-to-f-infty-o-infty