Answer
See the answer below.
Work Step by Step
$0\leq\ell\leq n-1$
$\begin{smallmatrix}
&n &\ell & Valid\ ?\\
\hline\\
3p&3&1&\surd\\
4s&4&0&\surd\\
2f&2&3&\times\\
1p&1&2&\times
\end{smallmatrix}$
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