I have the following
\begin{equation}
\begin{split}
\frac{1}{2}\sum_{i} z_i^2 + \sum_{i}s_it_i &= \frac{1}{2}\sum_{i}z_i^2 + \sum_{P_2}s_it_i+\sum_{R \setminus P_2}s_it_i\\
&= \frac{1}{2}\sum_i z_i^2 + \sum_{P_2}2(n-3-z_i) + \sum_{R \setminus P_2} (n-2-z_i)\\
&\ge \frac{1}{2}\sum_i z_i^2 - \sum_{R} z_i - \sum_{P_2}z_i+ 2(n-3)\sigma + (n-p-q-2)(n-2)\\
\end{split}
\end{equation}
Compiling gives me the error "Paragraph ended before split was complete", it mentions something about \par afterward. I can't spot any error.
%in that line. – Skillmon Aug 17 '20 at 18:46