 Lyapunov functions ${\rm {\mathcal L}}\left(Z_{R_{j} } \left(\pi '_{S} \right)\right)$ and ${\rm {\mathcal L}}\left(Z_{R_{j} } \left(\pi _{S} \right)\right)$, where $\pi '_{S} $ is a next entanglement swapping period, defined as $\Delta _{{\rm {\mathcal L}}} ={\rm {\mathcal L}}\left(Z_{R_{j} } \left(\pi '_{S} \right)\right)-{\rm {\mathcal L}}\left(Z_{R_{j} } \left(\pi _{S} \right)\right)$