Table columns widths are not equal

Question

I am trying two build a table, with the following code:

\begin{table}[]
\begin{tabular}{@{}lllll@{}}
\toprule
\multicolumn{3}{c}{gamma function} & \multicolumn{2}{c}{\begin{tabular}[c]{@{}c@{}}zeta and gamma \\ function\end{tabular}} \\ \midrule
1          & 1         & 1         & 1                                          & 1                                         \\
2          & 3         & 4         & 5                                          & 6                                         \\
4          & 7         & 8         & 43                                         & 21                                        \\ \bottomrule
\end{tabular}
\end{table}

But the third and fourth columns have different widths. I tried to set equal width for all columns, but doesn't work. How can I fix this problem?

Answer 1

Here's a solution which sets all five columns to have equal width. It employs the w column type that's provided by the array package. The column contents are center-set via the c option in the first argument of the w columns.

\documentclass{article}
\usepackage{booktabs}
\usepackage{array}  % for "w" column type
\begin{document}
\begin{table}
\begin{tabular}{@{} *{5}{w{c}{11mm} }@{}}
\toprule
\multicolumn{3}{@{}c}{gamma function} & 
\multicolumn{2}{c}{\begin{tabular}[c]{@{}c@{}}
   zeta and gamma \\ function\end{tabular}} \\ 
\midrule
1 & 1 & 1 & 1  & 1  \\
2 & 3 & 4 & 5  & 6  \\
4 & 7 & 8 & 43 & 21 \\ 
\bottomrule
\end{tabular}
\end{table}
\end{document}

Answer 2

I propose this, with an empty column to separate the groups, siunitx and makecell for the column heads:

\documentclass{article}
\usepackage{makecell} 
\usepackage{booktabs}
\usepackage{siunitx} 

\begin{document}

\begin{table}
\sisetup{table-format=2.0, table-number-alignment=center}
\begin{tabular}{@{}SSSc@{\enspace}SS}
\toprule
\multicolumn{3}{@{}c}{\thead{gamma \\ function}} & & \multicolumn{2}{@{\enspace}c}{\makebox[0pt]{\thead{zeta and\\ gamma \\ functions}}} \\
\midrule
1 & 1 & 1 & & 1 & 1 \\
2 & 3 & 4 & & 5 & 6 \\
4 & 7 & 8 & & 43 & 21 \\
\bottomrule
\end{tabular}
\end{table}

\end{document} 

Answer 3

The environment {NiceTabular} of nicematrix has a key columns-width=auto to enforce the same width to all the columns.

\documentclass{article}
\usepackage{nicematrix}
\usepackage{booktabs}
\usepackage{siunitx}

\begin{document}

\begin{NiceTabular}{@{}SSSSS@{}}[columns-width=auto]
\toprule
\Block{1-3}{gamma\\ function} &&& 
\Block{1-2}{zeta and\\ gamma\\ functions} \\
\midrule
1 & 1 & 1 & 1 & 1 \\
2 & 3 & 4 & 5 & 6 \\
4 & 7 & 8 & 43 & 21 \\
\bottomrule
\end{NiceTabular}

\end{document}