Solution (source code)

= Solution

The <first excited hydrogen eigenspace> is the four-dimensional $n=2$ space
$$
\boxed{
\{|2,0,0\rangle,\ |2,1,-1\rangle,\ |2,1,0\rangle,\ |2,1,1\rangle\}}.
$$
The first vector is the $2s$ state and the other three are the $2p$ states. All have the same unperturbed energy $E_2$.