# Distribution Theory/Support and singular support

**Definition (being zero on open sets)**:

Let be a smooth manifold, let be open, and let . Let be an open subset. We say that **is zero** on iff for all we have .

**Proposition (distribution is zero on union of opens where it is zero)**:

Let ( being a smooth manifold), and let . Suppose that is zero on a family of open subsets (). Then is also zero on

- .

**Proof:** Let . Then is a compact subset of . Hence, extract a finite subcover . Then pick a finite partition of unity on of functions that are subordinate to (using that is a compact subset of and convolving an indicator function on that with a mollifier of sufficiently small support), and use linearity of .

**Definition (support)**:

Let , where is an open subset of a smooth manifold. The **support** of is the set

- ,

where the union ranges over all open on which is zero.