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The divergence theorem for finite gluings of elementary solid regions
Statement
Let a finite gluing of elementary solid regions be given, with pieces , union and outer boundary presentation (Finite gluings of elementary solid regions and their outward boundary presentation), and let be a vector field on an open set containing . Then
the right-hand side being the flux of over .
The decomposition into pieces, the internal-or-outer designation of the patches and the pairing of the internal patches are hypotheses supplied with the gluing; nothing is asserted about a solid presented without them.
Facts & Assumptions
Given: The finite gluing with its pieces , their presentations , the internal-or-outer designations, the pairing involution, the union , the outer presentation , and the field on an open .
In a finite gluing the pieces are elementary solid regions with pairwise disjoint interiors whose union is , and the outer patches together form a compatible finite patch presentation of (Finite gluings of elementary solid regions and their outward boundary presentation).
The divergence of a field on an open subset of is (Divergence and curl of a vector field).
For a compatible finite patch presentation the oriented flux is the sum of the flux over its patches (Finitely patched regular surfaces, their area, scalar integrals, and flux), and (The Euclidean inner product on ).
For an elementary solid region with presentation and a field on an open set containing , (The divergence theorem on an elementary solid region).
For a finite gluing, is compact and Jordan measurable; for a continuous vector field on the union of the piece boundaries, the sum of the piece fluxes is the flux over the outer presentation; and for a continuous scalar function on , the sum of the piece volume integrals is the integral over the union (Internal faces cancel and volume integrals add when elementary solid regions are glued).
Proof
By [F1] each is an elementary solid region contained in , so is an open set containing and is on it; hence [L1] applies to each piece and gives for .
The function is continuous on by [F2], since a field has continuous first partial derivatives, and in particular continuous on .
Summing the identities of step 1.1 over and applying [L2] to each side — the volume clause with , continuous on by step 1.2, and the flux clause with , continuous on and on every — turns the left sum into and the right sum into , which by [F1] and [F3] is the flux over the outer boundary presentation of .
Step 2.1 is the asserted identity. The field is required to be on an open set containing the whole union, because step 1.1 applies the piecewise identity with that same field on each piece and step 2.1 integrates over .
Remarks
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What the gluing clause buys. A solid need not be simple in every coordinate direction: a U-shaped prism has sections in one direction that are unions of two disjoint intervals, so it admits no simple description there, and yet it is a gluing of three boxes. The companion examples page carries that computation.
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No connectedness is used. The pieces need not touch and the boundary need not be connected: step 2.1 rearranges finitely many real numbers and integrates over a finite union.
Depends on
- The divergence theorem on an elementary solid region
- Internal faces cancel and volume integrals add when elementary solid regions are glued
- Finite gluings of elementary solid regions and their outward boundary presentation
- Divergence and curl of a $C^1$ vector field
- Finitely patched regular surfaces, their area, scalar integrals, and flux
- The Euclidean inner product $\langle x,y\rangle = \sum_{k<n} x_k y_k$ on $\mathbb{R}^n$
Used by
- A field with vanishing divergence has zero outward flux through the boundary of a glued elementary solid Corollary
- Green's first identity on a glued elementary solid region Corollary
- Green's second identity on a glued elementary solid region Corollary
- The divergence at a point is the limit of outward flux per unit volume Corollary
- The flux of a curl through the boundary of a glued elementary solid vanishes Corollary
- The volume of a glued elementary solid is a third of the outward flux of the position field Corollary
- Vector forms: the boundary integrals of fn and of n× F Corollary
- A U-shaped prism is a finite gluing of three boxes and is not simple in every coordinate direction Example
- What the classical divergence and Stokes theorems here do and do not cover Remark
Dependency tree · two levels
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Sources
- J. Feldman, A. Rechnitzer and E. Yeager, CLP-4 Vector Calculus (University of British Columbia), Theorem 4.2.2 (standard reference, not scraped)
- G. Strang and E. Herman, Calculus Volume 3 (OpenStax), Theorem 6.20 (standard reference, not scraped)