Tricks

Problem 1: vector notation, bold instead of arrow

Solution: Add the following into your preamble:

\usepackage{amsmath} % used for boldsymbol.
\renewcommand{\vec}[1]{\boldsymbol{#1}} % Uncomment for BOLD vectors.

Problem 2: LaTeX adjusting sum limits

When you add sum into a complicated equation, the limits of sum may change its position, now you can add a limits, things will be good.

%type 1:
\begin{equation*}
p = \sum_{n=1}^Na_n
\end{equation*}

%type 2:
\begin{equation*}
p = \sum\limits_{n=1}^Na_n
\end{equation*}

%type 3:
\begin{equation*}
p = \sum\nolimits_{n=1}^Na_n
\end{equation*}