\begin{align*}
    \tilde f_i(\boldsymbol{X}) = \tilde f_{i-1}(\boldsymbol{X}) + \gamma_i\left(Y_i - \tilde f_{i-1}\left(\boldsymbol{X}_i\right)\right) \left(1+\sum_{k=1}^p\sum_{j=1}^{\infty}\psi_{jk}\left(X_i^{(k)}\right)\psi_{jk}\left(\boldsymbol{X}\right)\right),
\end{align*}
