MWE

\documentclass[10pt,a4paper]{article}
\usepackage{booktabs}
\begin{document}

\begin{table}[!htpb]
\centering
\begin{tabular}{ l l l }
\toprule
$\alpha$ & $Spec_\alpha (A)$ & $Spec_\alpha (B)$ \\
\midrule
0.95 & 0, 21, 22                 & 0 \\
0.9  & 0,  1, 21, 22, 42         & 0 \\
0.8  & 0,  1, 20, 21, 22, 23, 42 & 0 \\
0.5  & 0                         & 1 \\
\bottomrule
\end{tabular}
\end{table}

\end{document}