Proof that a curl-free vector field on a convex set is conservative

Builds on Curl Conservative vector field Convex set Differentiation under the integral sign Chain rule for a composition with a curve Continuous partial derivatives imply differentiability Gradient Product rule Fundamental theorem of calculus C^k function characterised by continuous derivatives Partial derivative

Proof

We differentiate the explicit formula for $\phi$ under the integral sign.

Write $F = (F_1, F_2, F_3)$ and $\partial_i = \partial / \partial x_i$ . By the definition of the curl, $\nabla \times F = 0$ means $\partial_2 F_3 = \partial_3 F_2$ , $\partial_3 F_1 = \partial_1 F_3$ and $\partial_1 F_2 = \partial_2 F_1$ . So $\partial_j F_i = \partial_i F_j$ for all $i, j$ .
Fix $p \in U$ . For $x \in U$ and $t \in [0, 1]$ , the point $p + t (x - p)$ lies in $U$ , since $U$ is convex. So $\phi(x) = \int_0^1 F(p + t (x - p)) \cdot (x - p) \, dt$ is defined, with a continuous integrand.
Each $F_i$ is $C^1$ , so it is differentiable by continuous partial derivatives imply differentiability. Its derivative in a direction $v$ is $\nabla F_i \cdot v$ , by the definition of the gradient.
Fix $x \in U$ and $j \in \set{1, 2, 3}$ . Choose $r > 0$ such that $x + h e_j \in U$ for $\abs{h} < 2 r$ , where $e_j$ is the $j$ -th basis vector.
For $t \in [0, 1]$ and $\abs{h} < 2 r$ , let $y_h = x + h e_j$ and $q(t, h) = p + t (y_h - p)$ . By step 2, $q(t, h) \in U$ .
Let $g(t, h) = F(q(t, h)) \cdot (y_h - p) = \sum_i F_i(q(t, h)) \, (y_{h, i} - p_i)$ . Then $\phi(x + h e_j) = \int_0^1 g(t, h) \, dt$ , and $g$ is continuous on $[0, 1] \times [-r, r]$ .
For fixed $t$ , the curve $h \mapsto q(t, h)$ has derivative $t e_j$ . By the chain rule along a curve and step 3, $\frac{\partial}{\partial h} F_i(q(t, h)) = t \, \partial_j F_i(q(t, h))$ .
Also $\frac{\partial}{\partial h} (y_{h, i} - p_i)$ is $1$ for $i = j$ and $0$ otherwise. By the product rule, $\frac{\partial g}{\partial h}(t, h) = t \sum_i \partial_j F_i(q(t, h)) \, (y_{h, i} - p_i) + F_j(q(t, h))$ .
This partial derivative is continuous on $[0, 1] \times [-r, r]$ , since the $\partial_j F_i$ are continuous. By differentiation under the integral sign at $h = 0$ , $\partial_j \phi(x) = \int_0^1 \frac{\partial g}{\partial h}(t, 0) \, dt$ .
Let $c(t) = p + t (x - p)$ , so $c(t) = q(t, 0)$ . By step 1, $\frac{\partial g}{\partial h}(t, 0) = t \sum_i \partial_i F_j(c(t)) \, (x_i - p_i) + F_j(c(t))$ .
The curve $c$ has derivative $x - p$ . By the chain rule along a curve and step 3, $\frac{d}{dt} F_j(c(t)) = \sum_i \partial_i F_j(c(t)) \, (x_i - p_i)$ for $t \in (0, 1)$ .
By the product rule and steps 10 and 11, $\frac{\partial g}{\partial h}(t, 0) = \frac{d}{dt} \big( t \, F_j(c(t)) \big)$ for $t \in (0, 1)$ . The function $t \mapsto t F_j(c(t))$ is continuous on $[0, 1]$ .
By the fundamental theorem of calculus, $\partial_j \phi(x) = 1 \cdot F_j(c(1)) - 0 \cdot F_j(c(0)) = F_j(x)$ .
So the partial derivatives of $\phi$ are $F_1, F_2, F_3$ , which are continuous. Hence $\phi$ is of class $C^1$ , and $\nabla \phi = F$ on $U$ by the coordinate form of the gradient.
Therefore $F$ is conservative on $U$ , with the potential $\phi$ of the statement.

Idea

The candidate potential is the work of $F$ along the straight segment from $p$ to $x$ . Differentiating in $x_j$ under the integral produces terms $\partial_j F_i$ . The curl-free condition turns them into $\partial_i F_j$ , and then the integrand becomes an exact $t$ -derivative of $t F_j$ , which integrates to $F_j(x)$ .

Reading path

To understand this page from the ground up, read these 359 pages in order. The foundations come first. The path takes the proofs Proof that Darboux and Riemann integrals agree, Proof of the limit of a sum of sequences, Proof of Rolle's theorem, Proof of the derivative of a map into R^n by components, Proof of the approximation property of the supremum, Proof that every sequence has a monotone subsequence, by peaks, Proof that open sets are the sets that are neighbourhoods of all their points, Proof of the commutativity of addition by induction, Proof of the Lebesgue number lemma by a finite cover of balls, Proof of the epsilon-delta form of continuity, Proof of the triangle inequality for norms, Proof of the Heine–Borel theorem by sequential compactness, Proof that there is a natural number n with 1/n below any positive epsilon, Proof of the chain rule in several variables, Proof of the derivative as a little-o approximation, Proof that continuous functions on a closed bounded interval are uniformly continuous, Proof that a linear map between normed spaces is continuous if and only if it is bounded, Proof that a function is bijective iff it has a two-sided inverse, Proof that the reals form an ordered field, with the Cauchy sequence construction, Proof of continuity via closed sets, Proof of the double negation law by a truth table, Proof of the extreme value theorem by the Bolzano–Weierstrass theorem, Proof that compact subsets of a metric space are bounded, Proof of differentiation under the integral sign, Proof that compactness and sequential compactness agree in metric spaces, Proof that subsequences of a convergent sequence converge to the same limit, Proof of the uniqueness of limits, Proof that the coordinate map is a linear isomorphism, Proof of strong induction from ordinary induction, Proof that linear maps on finite-dimensional normed spaces are continuous, Proof of the limit of a product of sequences, Proof that the absolute value is multiplicative, Proof of the squeeze theorem for sequences, Proof of the existence of square roots of nonnegative reals, Proof of the chain rule for a composition with a curve, Proof of the linearity of differentiation, Proof of the mean value theorem for integrals, Proof that a continuous image of a compact set is compact, Proof of the Heine–Cantor theorem by a finite cover of balls, Proof that compact subsets of a Hausdorff space are closed, Proof of epsilon-delta continuity at a point in metric spaces, Proof of Riemann's criterion for integrability, Proof of the sequential characterization of limits, Proof of the description of the open sets of a metric space, Proof of the triangle inequality for real numbers, Proof of the epsilon-N definition of the limit of a sequence, Proof of the first fundamental theorem of calculus, Proof that compactness of a subset can be tested with open sets of the ambient space, Proof of the existence of the infimum by negation, Proof of the equivalence of norms in finite dimension by compactness of the unit sphere, Proof that every spanning set contains a basis, by Zorn's lemma, Proof of Fermat's theorem on stationary points, Proof that limits preserve non-strict inequalities, Proof that closed sets contain their limits, Proof of the linearity of the integral, Proof of Zorn's lemma, Proof that metric spaces are Hausdorff, Proof that the derivative gives the directional derivatives, Proof of the second fundamental theorem of calculus, Proof of the intermediate value theorem by the supremum, Proof that differentiable functions are continuous, Proof of the sequence characterisation of uniform continuity, Proof of the epsilon-delta form of the limit of a function, Proof of the well-ordering principle from strong induction, Proof of the Bolzano–Weierstrass theorem by a monotone subsequence, Proof of the Archimedean property in an Archimedean ordered field, Proof that a continuous real function on a compact set attains its bounds, Proof of the derivative of a map into a product, Proof of the additivity of the integral over intervals, Proof that a decreasing sequence bounded below converges, Proof that the reals are Archimedean from the completeness axiom, Proof that convergent sequences are bounded, Proof of the product rule, Proof of the monotone convergence theorem for sequences, Proof that continuity is equivalent to sequential continuity, Proof that open balls are open, Proof of the monotonicity of the integral, Proof of the boundedness theorem by Bolzano–Weierstrass, Proof that Lipschitz maps are uniformly continuous, Proof that C^k functions are characterised by continuous derivatives, Proof that continuous partial derivatives imply differentiability, Proof that a set is open if and only if its complement is closed, Proof of the fundamental theorem of calculus, Proof of the mean value theorem from Rolle's theorem, Proof of continuity via open sets in metric spaces, Proof that continuous functions are Riemann integrable, Proof of the constant multiple rule.

Affine combination concept
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Complex numbers object
Construction of the reals from Cauchy sequences concept
Coordinate space object
Coordinate vector concept
Definition by recursion concept
Element definition
Euclidean metric example
Euclidean space object
Extended real number line object
Function concept
Function composition concept
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Limit point of a set definition
Linear isomorphism concept
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Local maximum definition
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Natural numbers object
Norm concept
Ordered field definition
Partition of an interval definition
Peano axioms concept
Proof of Zorn's lemma proof
Proposition concept
Rational numbers object
Real numbers object
Sequence definition
Set bounded above definition
Vector space definition
p-metric on R^n example
Real and imaginary parts definition
Convex set definition
Set definition
Fréchet derivative definition
Having a directional derivative definition
Total derivative concept
Function of several variables concept
Vector-valued function concept
Ring of integers statement
Limit of a function definition
Local minimum definition
Archimedean ordered field definition
Successor function definition
Mesh of a partition definition
Refinement of a partition definition
Riemann sum definition
Conjunction definition
Disjunction definition
Implication definition
Negation definition
Predicate definition
The reals form a field statement
Real sequence definition
Subsequence definition
Infimum definition
Supremum definition
Linear combination definition
Linear map definition
Linear subspace definition
Norm definition
p-norm on R^n example
Empty set object
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Ordered pair definition
Differentiable function of several variables concept
Partial derivative concept
Limit of a function of several variables concept
Parametrised curve concept
Intersection definition
Union definition
Continuity at a point definition
Derivative definition
Addition of natural numbers definition
Riemann integrable function definition
Proof of the Archimedean property in an Archimedean ordered field proof
Biconditional definition
The rationals form a field statement
The successor function is injective statement
Truth table concept
Zero is not a successor statement
Modus ponens concept
Existential quantifier definition
Universal quantifier definition
Bounded sequence definition
Limit of a sequence definition
Monotone sequence definition
Linear independence definition
Completeness axiom statement
Composition of linear maps definition
Linear isomorphism definition
Linear span definition
Multilinear map definition
Normed vector space definition
Cartesian product definition
Gradient concept
Jacobian matrix concept
Partial derivative definition
Continuity in R^n definition
Differentiable function of several variables definition
Continuous function definition
Union of a family of sets definition
Antiderivative definition
Differentiable function definition
Archimedean property statement
Riemann integral definition
Subset definition
Proof of the commutativity of addition by induction proof
Order on the natural numbers definition
Proof of the derivative as a little-o approximation proof
The rationals form an ordered field statement
Propositional logic concept
Mathematical proof concept
Uniqueness quantifier definition
Upper and lower Darboux sums definition
Convergent sequence definition
Sequential continuity definition
Spanning set definition
Higher partial derivatives definition
Differentiable on an open set definition
Binary relation definition
Gradient definition
Topological space definition
Proof of the derivative of a map into R^n by components proof
Universal set definition
Derivative as a little-o approximation statement
Order on the reals definition
Definite integral definition
Commutativity of addition statement
Proof of strong induction from ordinary induction proof
Tautology definition
Upper and lower Darboux integrals definition
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Basis definition
Iterated derivative definition
Antisymmetric relation definition
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Transitive relation definition
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Strong induction statement
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Proof of the epsilon-delta form of continuity proof
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Preimage of a set definition
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Proof of the well-ordering principle from strong induction proof
Proof of the double negation law by a truth table proof
Partial order definition
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Epsilon-delta definition of continuity statement
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Epsilon-delta definition of a limit statement
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Metric topology definition
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Proof by contradiction concept
Proof that every sequence has a monotone subsequence, by peaks proof
Chain definition
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Open set definition
Proof that a function is bijective iff it has a two-sided inverse proof
Proof that the reals form an ordered field, with the Cauchy sequence construction proof
Every sequence has a monotone subsequence statement
Hausdorff space definition
Bijective iff it has a two-sided inverse statement
Closed set definition
Proof that open sets are the sets that are neighbourhoods of all their points proof
Open cover definition
Continuous map of topological spaces definition
Proof that the derivative gives the directional derivatives proof
Zorn's lemma statement
Equivalent metrics definition
Proof of the description of the open sets of a metric space proof
The reals form an ordered field statement
Proof that a set is open if and only if its complement is closed proof
Compact space definition
Open sets characterised by neighbourhoods statement
Derivative gives the directional derivatives statement
Equivalent norms definition
Proof that the coordinate map is a linear isomorphism proof
Open sets of a metric space statement
Absolute value definition
Proof that every spanning set contains a basis, by Zorn's lemma proof
Proof of the approximation property of the supremum proof
Proof of the existence of square roots of nonnegative reals proof
Proof of the existence of the infimum by negation proof
Complement of an open set is closed statement
Compact set definition
Continuous linear map definition
Continuity at a point of a topological space definition
Proof that closed sets contain their limits proof
Proof of the triangle inequality for real numbers proof
Proof that the absolute value is multiplicative proof
Proof that open balls are open proof
Isomorphism with the coordinate space statement
Proof of the epsilon-N definition of the limit of a sequence proof
Every spanning set contains a basis statement
Existence of square roots of nonnegative reals statement
Approximation property of the supremum statement
Existence of the infimum statement
Proof that compactness of a subset can be tested with open sets of the ambient space proof
Proof of continuity via closed sets proof
Closed sets contain their limits statement
Triangle inequality statement
Absolute value is multiplicative statement
Open balls are open statement
Square root definition
Epsilon-N definition of the limit of a sequence statement
Proof of the intermediate value theorem by the supremum proof
Proof that the reals are Archimedean from the completeness axiom proof
Compactness via open covers statement
Modulus of a complex number definition
Continuity via closed sets statement
Proof of the limit of a sum of sequences proof
Proof of the uniqueness of limits proof
Proof that differentiable functions are continuous proof
Proof of epsilon-delta continuity at a point in metric spaces proof
Intermediate value theorem statement
Proof of continuity via open sets in metric spaces proof
Proof of the squeeze theorem for sequences proof
Proof that limits preserve non-strict inequalities proof
Proof of the Lebesgue number lemma by a finite cover of balls proof
Proof that metric spaces are Hausdorff proof
Proof of the linearity of the integral proof
Proof that convergent sequences are bounded proof
The reals are Archimedean statement
Proof that subsequences of a convergent sequence converge to the same limit proof
Proof of the monotone convergence theorem for sequences proof
Proof that compact subsets of a Hausdorff space are closed proof
Proof that a continuous image of a compact set is compact proof
Proof that compact subsets of a metric space are bounded proof
Epsilon-delta continuity in metric spaces statement
Lebesgue number lemma statement
Metric spaces are Hausdorff statement
Differentiable functions are continuous statement
Proof that compactness and sequential compactness agree in metric spaces proof
Proof of the triangle inequality for norms proof
Limit of a sum of sequences statement
Proof of the sequence characterisation of uniform continuity proof
Uniqueness of limits statement
Linearity of the integral statement
Proof of the monotonicity of the integral proof
Limits preserve non-strict inequalities statement
Proof that there is a natural number n with 1/n below any positive epsilon proof
Convergent sequences are bounded statement
Squeeze theorem statement
Proof that Darboux and Riemann integrals agree proof
Subsequences of a convergent sequence converge to the same limit statement
Monotone convergence theorem for sequences statement
Compact subsets of a Hausdorff space are closed statement
Continuous image of a compact set is compact statement
Compactness and sequential compactness agree in metric spaces statement
Compact subsets of a metric space are bounded statement
Continuity via open sets statement
Proof of the limit of a product of sequences proof
Epsilon-delta uniform continuity statement
Monotonicity of the integral statement
Darboux and Riemann integrals agree statement
There is a natural number n with 1/n below any positive epsilon statement
Triangle inequality for norms statement
Proof that a decreasing sequence bounded below converges proof
Proof that a linear map between normed spaces is continuous if and only if it is bounded proof
Proof of the Heine–Cantor theorem by a finite cover of balls proof
Proof that Lipschitz maps are uniformly continuous proof
Proof of the sequential characterization of limits proof
Proof that continuity is equivalent to sequential continuity proof
Limit of a product of sequences statement
Decreasing sequence bounded below converges statement
Proof of Riemann's criterion for integrability proof
Heine–Cantor theorem statement
Lipschitz maps are uniformly continuous statement
Continuity is equivalent to sequential continuity statement
Riemann's criterion for integrability statement
Sequential characterization of limits statement
Proof of the Bolzano–Weierstrass theorem by a monotone subsequence proof
Bounded linear maps are continuous statement
Proof of the product rule proof
Proof of the linearity of differentiation proof
Proof of the additivity of the integral over intervals proof
Bolzano–Weierstrass theorem statement
Proof of the derivative of a map into a product proof
Product rule statement
Additivity of the integral over intervals statement
Linearity of differentiation statement
Proof of the Heine–Borel theorem by sequential compactness proof
Proof that continuous functions on a closed bounded interval are uniformly continuous proof
Proof of the boundedness theorem by Bolzano–Weierstrass proof
Derivative of a map into a product statement
Proof of the constant multiple rule proof
Continuous functions on a closed bounded interval are uniformly continuous statement
Heine–Borel theorem statement
Boundedness theorem statement
Proof of the first fundamental theorem of calculus proof
Constant multiple rule statement
Derivative of a map into R^n by components statement
Proof that a continuous real function on a compact set attains its bounds proof
Proof of the extreme value theorem by the Bolzano–Weierstrass theorem proof
Extreme value theorem statement
Continuous real function on a compact set attains its bounds statement
Proof of Fermat's theorem on stationary points proof
Proof that continuous functions are Riemann integrable proof
Proof of the equivalence of norms in finite dimension by compactness of the unit sphere proof
Fermat's theorem on stationary points statement
Continuous functions are Riemann integrable statement
Proof of Rolle's theorem proof
Equivalence of norms in finite dimension statement
Rolle's theorem statement
Proof that linear maps on finite-dimensional normed spaces are continuous proof
Proof of the mean value theorem for integrals proof
Linear maps on finite-dimensional spaces are continuous statement
Mean value theorem for integrals statement
Proof of the mean value theorem from Rolle's theorem proof
Mean value theorem statement
First fundamental theorem of calculus statement
Proof of the chain rule in several variables proof
Proof of differentiation under the integral sign proof
Proof that continuous partial derivatives imply differentiability proof
Chain rule in several variables statement
Proof of the second fundamental theorem of calculus proof
Differentiation under the integral sign statement
Continuous partial derivatives imply differentiability statement
Proof of the chain rule for a composition with a curve proof
Second fundamental theorem of calculus statement
Proof of the fundamental theorem of calculus proof
Chain rule for a composition with a curve statement
Proof that C^k functions are characterised by continuous derivatives proof
C^k function characterised by continuous derivatives statement
Fundamental theorem of calculus statement
Vector field definition
Conservative vector field definition
Curl definition
Proof that a curl-free vector field on a convex set is conservative proof

Where this leads

5 pages build on this one.

Tag	`242AXRRT28`
URI	`m2://242AXRRT28`
Kind	proof
Writing	W0 stub
MSC	26B12
Proves	Curl-free vector field on a convex set is conservative
Uses	Curl, Conservative vector field, Convex set, Differentiation under the integral sign, Chain rule for a composition with a curve, Continuous partial derivatives imply differentiability, Gradient, Product rule, Fundamental theorem of calculus, C^k function characterised by continuous derivatives, Partial derivative
Changed	4 November 2026

Proof that a curl-free vector field on a convex set is conservative

Proof

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