\input{_preamble}
\begin{document}
    \myeqs{
        \myeqs{
            \mathcal{N}_{i,p-k}^{\boldsymbol{U}^{(k)}}(u)
            = \mu_{i,p-k-1}^{(k)}(u)\, \mathcal{N}_{i,p-k-1}^{\boldsymbol{U}^{(k)}}(u)
            + \left[1-\mu_{i+1,p-k-1}^{(k)}(u)\right]\, \mathcal{N}_{i+1,p-k-1}^{\boldsymbol{U}^{(k)}}(u)
        }
    }
\end{document}