I want to draw a rectangle commutative diagram, but I do not know how. The MathJax codes I find on the internet do not work here. I tried the following:
$\xymatrix
\mathbb{Z}/p^n\mathbb{Z}\ar[r]^{\theta_{n+1,\alpha}} \ar[d] U_1/U_{n+1}\ar[d] \\
\mathbb{Z}/p^{n-1}\mathbb{Z} \ar[r]^{\theta_{n,\alpha}} \\
U_1/U_n
$