“…(The diffusion constant may be discontinuous across the interface Σ; this case is treated below.) If (x i−1 , y j ), (x i , y j ), and (x i+1 , y j ) are all in the same region, i.e., (19) or (20) then we can use the standard second-order discretization (21) Suppose, however, that (x i , y j ) and (x i+1 , y j ) are not in the same region. Assume without loss of generality that (x i , y j ) ∈ Ω and (x i+1 , y j ) ∈ Ω c ; the case where (x i , y j ) ∈ Ω c and (x i+1 , y j ) ∈ Ω is treated similarly.…”