I would like to center the tikzpicture in first column. In the output, the graph alignment is different for rows.
\documentclass[12pt]{report}
\renewcommand{\baselinestretch}{1.5}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\tikzstyle{vertex}=[circle, draw, inner sep=0pt, minimum size=6pt]
\newcommand{\vertex}{\node[vertex]}
\newcounter{Angle}
\textwidth 5.55in
\textheight 7.35in
\begin{document}
\[\begin{array}{|b|lc|lc|lc|}
\hline
G & \multicolumn{2}{c}{cr(G+nK_{1})} \vline & \multicolumn{2}{c|}{cr(G+P_{n})} & \multicolumn{2}{c|} {cr(G+C_{n})} \\
\hline
%enter first row%
\begin{tikzpicture}[x=0.7cm, y=0.7cm]
\vertex (a) at (0,0) {};
\vertex (b) at (1,0) {};
\vertex (c) at (0,1) {};
\vertex (d) at (1,1) {};
\end{tikzpicture}& Z(4,n) & n\geq 1 & Z(4,n) & n\geq 1 & Z(4,n) & n\geq 3 \\ \hline
%%%%Entering second row%%%%
\begin{tikzpicture}[x=0.7cm, y=0.7cm]
\vertex (a) at (0,0) {};
\vertex (b) at (1,0) {};
\vertex (c) at (0,1) {};
\vertex (d) at (1,1) {};
\path
(a) edge (c)
\end{tikzpicture}
& Z(4,n) & n\geq 1 & Z(4,n) & n\geq 1 & Z(4,n) & n\geq 3 \\ \hline
%%%%Entering third row%%%%
\begin{tikzpicture}[x=0.7cm, y=0.7cm]
\vertex (a) at (0,0) {};
\vertex (b) at (1,0) {};
\vertex (c) at (0,1) {};
\vertex (d) at (1,1) {};
\path
(a) edge (c)
(b) edge (d)
\end{tikzpicture}
& Z(4,n) & n\geq 1 & Z(4,n) & n\geq 1 & Z(4,n) & n\geq 3 \\ \hline
\end{array}\]
\]
\begin{center}
\caption {Summary of crossing numbers for $G+nK_{1}$, $G+P_{n}$ and $G+C_{n}$.}
\end{center}
\end{document}


dvipsdriver with TikZ which will eventually lead to problems. (3) The compilation routes forpstricksand TikZ can often be at odds with one another. (4) Math environments do not like empty lines in them. – A.Ellett Dec 16 '13 at 05:13