ex4.tex

\input arrows.tex
$$\vcenter {\offinterlineskip
   \def \cc#1{ \hfil #1 \hfil }
   \halign {
   % preamble
      \cc {$#$}& #\tv && \cc {$#$}
   \cr
      x&& -\infty && 0&& +\infty
   \cr
   \noalign {\hrule height 1pt }
      f' (x)&& &-& 0& +
   \cr
   \noalign {\hrule height 1pt}
      \buucenter {$f (x)$}&& &
      \brightddownarrow & \down{$0$}& 
      \brightuuparrow & \buup {\phantom {1}}
   \cr
}}
$$
\bye