\begin{subequations}
\begin{equation}
    P_{\rm KRR}^{(3)}\left(\boldsymbol{X}_q\right) := \sum_{i_3=1}^{N^{(3)}_{\rm train}}\alpha^{(3)}_{i_3}k\left(\boldsymbol{X}_{i_3},\boldsymbol{X}_q\right)
\end{equation}
\begin{equation}
    P_{\rm KRR}^{(3,4)}\left(\boldsymbol{X}_q\right) := \sum_{i_4=1}^{N^{(4)}_{\rm train}}\alpha^{(3,4)}_{i_4}k\left(\boldsymbol{X}_{i_4},\boldsymbol{X}_q\right)
\end{equation}
\begin{equation}
    P_{\rm KRR}^{(4,5)}\left(\boldsymbol{X}_q\right) := \sum_{i_5=1}^{N^{(5)}_{\rm train}}\alpha^{(4,5)}_{i_5}k\left(\boldsymbol{X}_{i_5},\boldsymbol{X}_q\right)
\end{equation}
\end{subequations}
