With the code:
$$A=\msq{\mat{ccccc}
2 & -1 & & \multi[2][2]{\Huge{$0$}} \\
-1& 2 & -1 \\
& \ddots & \ddots & \ddots \\
\multi[2][2]{\Huge{$0$}} & \ddots & \ddots & \ddots \\
& & & -1 & 2
\emat}.$$
I get:
Overfull \vbox (6.0pt too high) detected at line 45
Overfull \vbox (6.0pt too high) detected at line 45
where the code ends at line 45. How can I fix that?
NB \multi is as in Problem with abbreviation of `\multirow` and `\multicolumn` (LaTeX), \msq is \msq{#1}=\left[\!\!\!\!#1\!\!\!\!\right] and \mat=\begin{array}, \emat=end{array}.
\multiand\emat? Have you seen Why is\[…\]preferable to$$?? Instead of a code snippet like this, it is always better provide full, compilable code in the form of a minimal working example (MWE).$$should not be used in LaTeX, thus it might be plain TeX.\Hugemight be a hint for LaTeX.\msq,\mat#1#2\emat,\multi[#1][#2]#3seem to be private macros. A MWE clarifies all these issues.\multi[2][2]{\Huge{$0$}}does. Probably a fixed height vertical box, which a\Hugezero can't fit in. Use\hugeor a smaller size.\[\]instead of$$. I have seen tex.stackexchange.com/questions/503/why-is-preferable-to many times, but 1) I can't get used to the change because$$is automatic and I need speed typing in notes, and 2) it seems from that question that the difference is seen only in a few rare cases and is rarely any big issue, correct me if I'm wrong, so I don't see why I should change.\smash{\Huge{$0$}}to make the "apparent" height and depth zero. you might have to fiddle with the vertical position. (sorry; this is unchecked, so it might not really work as i expect.)