“…Optimized Schwarz methods are a modern class of Schwarz methods which use instead of the classical Dirichlet transmission conditions at the interfaces more effective transmission conditions, which can take the physics of the problem at hand into account, see [18,19] and references therein. This property is especially important for anisotropic diffusion problems, which behave very differently at interfaces depending on the orientation of the diffusion, see for example [24], [14,Section 5], and the very recent reference [35]; for classical Schwarz methods applied to anisotropic diffusion, see [36,8,11], and for a specific earlier two level preconditioner [32]. Similarly when discretizing anisotropic diffusion problems, the numerical scheme must be suitable for high anisotropy, and discrete duality finite volume (DDFV) methods have this property, even in the case of discontinuous anisotropic diffusion, see [27,5,26,6,9,15,2], and in particular [16,Part II] which is dedicated especially to anisotropic diffusion.…”