Alphabeta Math

Line Integrals and the Gradient Theorem

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1 · Prerequisites
Abstract Algebra
Combinatorics
Coxeter Groups
Linear Algebra
Real Analysis
Set Theory
Topology
2 · Logical flowchart

3 · Items

Definition

Reversal, concatenation, closed paths, and oriented piecewise-C1 reparametrizations

Definition

Scalar line integrals with respect to arc length and vector-field line integrals

Lemma

The piecewise-C1 line-integral sums do not depend on the admissible partition

Theorem

Scalar line integrals are parametrization-independent; vector line integrals retain orientation and change sign when it reverses

Theorem

Line integrals under reversal and concatenation

Theorem

Line-integral estimates by arc length and the supremum of the field

Corollary

The scalar line integral of one is the arc length

Corollary

For a C1 path the arc-length accumulation function has derivative equal to speed

Definition

Piecewise-C1 path-connected domains, potential functions, conservative fields, and path independence

Theorem

The gradient theorem: the line integral of a gradient is the endpoint increment

Corollary

Conservative fields are path-independent and have zero integral around every closed path

Theorem

Path independence is equivalent to zero integral around every closed piecewise-C1 path

Theorem

A continuous path-independent field has a potential constructed by line integrals

Theorem

Conservative, path-independent, and zero-closed-loop conditions are equivalent

Corollary

Two potentials of the same field differ by a constant on each piecewise-C1 path component

Lemma

Potentials glue after a constant adjustment over a nonempty path-connected overlap

Definition

Exact and closed C1 vector fields

Lemma

Continuous second partials of a scalar potential commute

Theorem

Every exact C1 vector field is closed

Definition

Star-shaped open subsets of Euclidean space

Theorem

Poincare's lemma on a star-shaped domain: every closed C1 field is exact

Corollary

On a star-shaped open domain, closed, exact, conservative, path-independent, and zero-loop are equivalent

Definition

Type I, Type II, and elementary regions for Green's theorem

Definition

Positive orientation of elementary-region boundaries

Lemma

The Type I boundary identity for the P dx term

Lemma

The Type II boundary identity for the Q dy term

Lemma

Shared boundary arcs cancel when finitely many elementary regions are glued

Theorem

Green's theorem for finite unions of elementary regions

Corollary

Area of an elementary Green region as a boundary line integral

Remark

Limitation: arbitrary Jordan domains are not covered by the elementary Green theorem

Remark

Closedness is local, exactness is global, and a domain hypothesis cannot be omitted

Sources