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]