\begin{figure}[h]
    \centering
  \begin{subfigure}{0.28\textwidth}
    \centering
    \includegraphics[width=\linewidth]{figure/M_4001.png}
    \caption{Different numbers of RFSs on MovieLens.}
  \end{subfigure}
    \hspace{0.6cm}
  \begin{subfigure}{0.286\textwidth}
    \centering
    
    \includegraphics[width=\linewidth]{figure/A_4001.png}
    \caption{Different numbers of RFSs on Avazu.}
    \label{25}
  \end{subfigure}
  \caption{Comparison of using fixed number (light blue) and decreasing numbers (dark blue) of RFS.}
\label{Fourier2}
\end{figure}