Heterogeneous problems and partition of unity decompositionWe are interested in solving linear PDEs of the formwhere Ω is a bounded domain in R d with d = 1, 2, L is a linear (elliptic) differential operator, f and g are the data, and u is the solution to (1). The weak form of (1) with a Hilbert spacewhereis the bilinear form corresponding to the operator L , and : V → R is the linear functional induced by f . We assume that (2) has a unique solution u ∈ {v ∈ V : v = g on ∂ Ω }, and that u is "heterogeneous", behaving very differently in different parts of Ω . Typical examples are advection-diffusion problems, where there are advection dominated and diffusion dominated regions (subdomains), and the boundaries in between are not clearly defined, see [8,10] and references therein. Apart from the χ-method [6, 1], there are no methods to determine such subdomain decompositions, and our goal is to present and study a new such method. We thus introduce (see [18,11])Given two membership functions ϕ 1 , ϕ 2 : Ω → [0, 1] that form a partition of unity on Ω , ϕ 1 (x) + ϕ 2 (x) = 1 for all x ∈ Ω , their supports provide then a domain decompo-