\begin{table}%t2
      \caption{Iteration information of the GMRES($m$) solver with $m=50$ and various settings of $\varepsilon$ ($\varepsilon_1=10^{-16}$, $\varepsilon_2=10^{-6}$, $\varepsilon_3=10^{-2}$).}
      \label{t:perfor:step}
      %\centering
\par
      \begin{tabular}{llll}
      \hline
      \hline
DOF $(N)$ &    GMRES($50,\varepsilon_1$) &   GMRES($50,\varepsilon_2$)   &     GMRES($50,\varepsilon_3$)\\
\hline
1023    &   (129,   134,   137)   &    (35,    41,    42) &     (6,    11,    11)\\
1443    &   (140,   145,   148)   &    (38,    43,    45) &     (6,    11,    11)\\
1935    &   (149,   151,   173)   &    (40,    45,    47) &     (6,    11,    11)\\
2499    &   (172,   175,   176)   &    (42,    47,    49) &     (6,    11,    11)\\
3135    &   (178,   183,   182)   &    (43,    50,    53) &     (5,    11,    11)\\
3843    &   (186,   194,   194)   &    (45,    51,    59) &     (5,    11,    11)\\
4623    &   (193,   200,   201)   &    (46,    56,    61) &     (5,    11,    11)\\
5475    &   (200,   228,   230)   &    (47,    59,    63) &     (5,    11,    11)\\
6399    &   (204,   230,   233)   &    (48,    60,    65) &     (5,    11,    11)\\
7395    &   (230,   236,   238)   &    (49,    64,    67) &     (5,    11,    11)\\
\hline
\end{tabular}
\end{table}