Fig. 1. The nonequilibrium double quantum-dot system with Coulomb coupling.

Fig. 2. The states and transition modes of the double quantum-dot system with Coulomb coupling.

Fig. 3. The directed graph of the four-state model and its cycle basis.

Fig. 4. (a) The entropy production rates , , and , and (b) the curves of the electron flows and varying with the Coulomb coupling strength, where , , , , and

Fig. 5. The curves of the partial entropy production rates , , and varying with the Coulomb coupling strength.

Fig. 6. The curves of the coarse-grained entropy production rates varying with the Coulomb coupling strength.