[Tex/LaTex] abovedisplayskip vs abovedisplayshortskip

Question

What is the difference between these two?

http://en.wikibooks.org/wiki/TeX/abovedisplayskip

http://en.wikibooks.org/wiki/TeX/abovedisplayshortskip

I don't know why, but it seems to work unusually.

\setlength{\abovedisplayskip}{5pt}
\setlength{\belowdisplayskip}{50pt}
\setlength{\abovedisplayshortskip}{2pt}
\setlength{\belowdisplayshortskip}{2pt} 

\begin{equation}
u_{i,t} = u_{i,t-1} + v_t
\end{equation}
\begin{equation}
u_{i,t-1} = u_{i,t-2} + v_{t-1}
\end{equation}
\begin{equation}
\Delta u_{it} = u_{i,t} - u_{i,t-1} = u_{i,t-1} + v_t - u_{i,t-1} =
v_t
\end{equation}

So why does it look like this:

I expected the short display skip to be between consecutive formulae and the regular skip to be before and after the first and last in the series, or something like that. In any case, why the heck would it do this?

Answer 1

Others have well described when the "short" skips are selected; as to why, it's easy: the purpose is avoiding large gaps between text and a centered equation.

How to avoid unbalanced spacing before and after equations? The answer is simple: use amsmath that provides many environments in order to accommodate multiple equations or alignments. For example, your case study should be input as

\begin{gather}
u_{i,t} = u_{i,t-1} + v_t \\
u_{i,t-1} = u_{i,t-2} + v_{t-1} \\
\Delta u_{it} = u_{i,t} - u_{i,t-1} = u_{i,t-1} + v_t - u_{i,t-1} =
v_t
\end{gather}

which avoids the "unbalancedness" and also prevents bad page breaks. By default amsmath will never allow a display environment to be broken across pages, but it's possible to override locally this behavior at final revision time with a suitably placed \displaybreak command.