\begin{align}
\begin{aligned}
\label{eq:X1}
\lVert(I_k+E_{1,1})(Q_1+\Delta_{1,1})+E_{1,2}\Delta_{2,1}-Q_1\rVert_2
\leq\lVert E_{1,1}\rVert_2+\lVert E_{1,2}\rVert_2\lVert \Delta_{2,1}\rVert_2
=O(\tilde\eta^2),
\end{aligned}
\end{align}
