Diffusion in non-equilibrium phenomena is typical and universal. Any unbalance away from the equilibrium is always/necessarily relaxed to the steady state. Here non-uniform (periodic) modulation of the diffusion constant opens up an energy gap in the spectrum for the bulk (without boundaries). Then with boundaries, there exist localized modes as edge states protected by the non-trivial bulk gap. In our recent paper “Bulk-edge correspondence of classical diffusion phenomena” by Tsuneya Yoshida and Yasuhiro Hatsugai, published in Scientific Reports 11, 888 (2021), we have proposed this bulk-edge correspondence for diffusive phenomena and demonstrated in 1D and 2D honeycomb lattice systems. See also arXiv:2007.08730.