This is LaTeX code:

[latex]
\\ch z =1+\frac{z^2}{2!}+\frac{z^4}{4!}+\cdots=\sum_{k\geq 0}\underbrace{\frac{z^{2k}}{(2k)!}}_{t(k)}=F\left(
\begin{array}{c|c}
 & \underline{z^2} \\
 \frac{1}{2} & 4 \\
\end{array}
\right)
[/latex]