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Smooth local continuity equation and its integrated forms
Statement
Assume the Axiom of Countable Choice (The Axiom of Countable Choice ()). Fix , , let and be nonempty open sets, let be continuous and let be such that , and all spatial partial derivatives , () exist and are continuous on , and where is the spatial gradient and is the classical Laplacian on (The Laplacian of a function and of a vector field, Directional derivatives and partial derivatives of a map ). Define the current Then:
(a) pointwise continuity equation. For every , where the divergence is the classical one of the field (Divergence and curl are linear and satisfy the scalar product rules);
(b) integrated balance. If is a bounded open set with boundary and (Divergence on a bounded C1 Euclidean domain), then is differentiable on with the outward unit normal;
(c) global conservation under domination. If , if there is with for almost every and every , and if then, for all such that the closed interval joining and is contained in ,
(d) norm conservation from unitary dynamics. Assume additionally the Axiom of Choice (The Axiom of Choice). If is a self-adjoint operator on a complex Hilbert space and for some , then for every (A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).
Assertion (d) is independent of the smooth local model of (a)-(c); no interpretation of as a measured counting rate is asserted.
Facts & Assumptions
Given: , open , nonempty open time set , continuous real , a solution of the displayed equation with continuous , and the current .
The real one-variable product and sum rules are supplied by Sums, scalar multiples, products and quotients: , , , and when . Writing with real and restricting to a time or spatial coordinate line gives and likewise for . Also ; the real scalar-vector product rule of Divergence and curl are linear and satisfy the scalar product rules gives , since the two gradient inner products cancel.
The conjugated equation: , since is real-valued.
Divergence theorem: for a bounded domain and , (Divergence on a bounded C1 Euclidean domain).
Differentiation under the integral sign on an open interval : if has integrable spatial sections, differentiable time sections outside one measurable null set, measurable spatial sections of its time derivative, and outside one measurable null set for every , with one nonnegative , then is differentiable on with derivative (Differentiation under the integral sign).
Dominated convergence, and the fact that an integral over of an class converges to the integral over as (Dominated convergence).
A self-adjoint with spectral PVM has , a strongly continuous unitary group: each is unitary, hence norm preserving (A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).
If a real function is continuous on , differentiable on , and there, the mean value theorem gives (The mean value theorem, as the case of Cauchy's: for continuous on with and differentiable on there is with ).
Proof
[The pointwise identity] At a point , [F2] gives and hence, by [F1], , because the term is purely imaginary and is real. Comparing with [F1]'s expression for gives , which is (a).
[The integrated balance] Let be as in (b). If , then , both integrals are zero and (b) holds. Suppose and fix . Openness of gives with ; put . By step 1.1, , continuous on the nonempty compact set , so . Each section is bounded on and integrable on the bounded set ; its time derivative exists everywhere and has continuous, hence measurable, spatial sections. The fixed majorant on is integrable and bounds the derivative for every , with no exceptional set. Thus [F4] applies to , giving differentiability at and derivative . By [F3] applied to the field , this equals . Since was arbitrary, (b) holds on the possibly disconnected open set .
[Global conservation] Assume (c), and let with and . For with we have by step 2.1 applied to (for so large that , which always holds) , so [F7] gives , and the right side tends to as by the hypothesis on the flux. By [F5], for and , using . Letting in the inequality gives . Thus the norms agree for every pair with the closed interval between them contained in , proving (c).
[Unitary dynamics] By [F6], with unitary, so for every . This is (d) and completes the proof.
Depends on
Differentiation under the integral sign
Divergence on a bounded C1 Euclidean domain
Dominated convergence
Sums, scalar multiples, products and quotients: , , , and when
The chain rule for total derivatives:
The mean value theorem, as the case of Cauchy's: for continuous on with and differentiable on there is with
Directional derivatives and partial derivatives of a map
The Laplacian of a function and of a vector field
Divergence and curl are linear and satisfy the scalar product rules
A self-adjoint operator generates a strongly continuous unitary group
Integral of a measurable function against a projection-valued measure
Complex Lp classes and Euclidean test-function conventions
The space as the quotient by null functions
The Axiom of Countable Choice ()
The Axiom of Choice