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Divergence on a bounded C1 Euclidean domain
Statement
Assume . For , a bounded domain Omega and , . Both integrals are finite, with the continuous interior derivative convention and the outward normal on every boundary component.
Facts & Assumptions
Given: Assume , , a bounded domain Omega, and with the continuous interior derivative convention.
A compact Euclidean set admits a finite subordinate ambient partition. (Finite ambient partitions near compact sets).
Localized graph fields satisfy the flux identity and interior fields have zero integral divergence. (The local graph flux calculation).
Graph density and continuous outward normal are independent of charts. (Chart and partition independence of surface measure).
Finite sums of integrable functions have the sum of their integrals. (The Lebesgue integral is linear on ).
A rigid coordinate change preserves volume integrals, by Borel substitution with determinant modulus one, applied to positive and negative parts. (Borel change of variables from the compact-support formula and Radon uniqueness).
Proof
Compactness of the boundary gives finitely many smaller graph cylinders covering it. Together with the open set Omega these cover the compact closure of Omega. F1 gives an ambient partition chi_j subordinate to this cover. For each boundary term chi_j F, F2 and F3 identify its divergence integral with its outward surface flux, since . The interior term has divergence integral zero and boundary trace zero by F2. Rigid coordinate changes preserve this calculation: the transformed field is , its derivative is and has the same trace, its dot products are unchanged, and its volume Jacobian has modulus one.
The continuous F and DF are bounded on the compact closure; Omega is bounded of finite volume and F3 gives finite boundary area. Thus all terms are integrable. The product rule gives , because the partition sum is one on a neighborhood of the closure. The boundary flux sum likewise equals . F4 sums the local identities from step 1.1 to give the stated theorem.
Source notes
Hunter §1.12 Theorem 1.46, printed pp. 17–18; Oh §3.9 Proposition 3.23, printed/PDF pp. 47–48, for the local-to-global proof.
Depends on
- Chart and partition independence of surface measure
- Finite ambient partitions near compact sets
- The local graph flux calculation
- The Lebesgue integral is linear on $L^1(\mu)$
- Sums, scalar multiples, products and quotients: $(f+g)'(c) = f'(c) + g'(c)$, $(\alpha f)'(c) = \alpha f'(c)$, $(fg)'(c) = f'(c)g(c) + f(c)g'(c)$, and $(f/g)'(c) = \bigl(f'(c)g(c) - f(c)g'(c)\bigr)/g(c)^{2}$ when $g(c) \ne 0$
- Borel change of variables from the compact-support formula and Radon uniqueness
Used by
- First Green identity Corollary
- The wrong normal gives the wrong sign Counterexample
- Flux and scaling on balls Example
- Euclidean divergence and the Stokes comparison Remark
Dependency tree · two levels
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Sources
- Hunter, Notes on Partial Differential Equations (standard reference, not scraped)
- Sung-Jin Oh, Lecture Notes for Math 222A (standard reference, not scraped)