Global analysis, analysis on manifolds Proof

Proof that the exterior derivative squares to zero

Builds on Exterior derivative Wedge product of forms Differential form Schwarz's theorem for C^2 functions C^k function characterised by continuous derivatives Sign of a permutation Sign is a homomorphism Transposition

Proof

We expand $d(d\omega)$ in coordinates. The mixed second derivatives are symmetric, while the wedge symbols are antisymmetric, so the terms cancel in pairs.

Write $\partial_j = \partial / \partial x_j$ . For a finite sequence $J = (j_1, \dots, j_m)$ of indices in $\set{1, \dots, n}$ , let $e_J = dx_{j_1} \wedge \dots \wedge dx_{j_m}$ , as in the definition of the wedge product. It is $0$ if two indices are equal. Otherwise it is $\operatorname{sgn}(\sigma) \, dx_K$ , where $K$ is the increasing rearrangement and $\sigma$ the sorting permutation.
Fact A: let $J$ have no repeated index, with $e_J = \varepsilon \, dx_K$ . Then $e_{(l, J)} = \varepsilon \, e_{(l, K)}$ for each index $l$ .
Proof of fact A. If $l$ occurs in $J$ , then it occurs in $K$ , and both sides are $0$ . Otherwise, sort $(l, J)$ in two stages: first sort the last $m$ places, turning $J$ into $K$ , then sort $(l, K)$ .
The first stage has sign $\varepsilon$ . The sign is multiplicative, because the sign is a homomorphism. So the sign of sorting $(l, J)$ is $\varepsilon$ times the sign of sorting $(l, K)$ , which is fact A.
Fact B: for indices $l, j$ and a sequence $I$ , $e_{(l, j) I} = -e_{(j, l) I}$ , where $(l, j) I$ means $(l, j, i_1, \dots, i_k)$ . If there is a repeated index, both sides are $0$ .
Proof of fact B without repeats. The two sequences differ by the transposition of the first two places. So their sorting permutations differ by a transposition, which has sign $-1$ , and the homomorphism property gives the sign change.
Let $\omega = \sum_I f_I \, dx_I$ be a $k$ -form whose coefficients $f_I$ are of class $C^2$ . By the definition of the exterior derivative, and since $dx_j \wedge dx_I = e_{(j) I}$ , we have $d\omega = \sum_I \sum_j \partial_j f_I \, e_{(j) I}$ .
For $j \notin I$ , write $e_{(j) I} = \varepsilon(j, I) \, dx_{K(j, I)}$ with $\varepsilon(j, I) = \pm 1$ . For $j \in I$ , $e_{(j) I} = 0$ . Collecting terms gives $d\omega = \sum_K g_K \, dx_K$ , where $g_K$ is the sum of $\varepsilon(j, I) \, \partial_j f_I$ over the pairs with $j \notin I$ and $K(j, I) = K$ .
Each $g_K$ is a finite sum of first partial derivatives of $C^2$ functions, so it is $C^1$ . So $d(d\omega)$ is defined, and $d(d\omega) = \sum_K \sum_l \partial_l g_K \, e_{(l) K}$ .
Expanding $g_K$ by step 8 and regrouping the finite sum, $d(d\omega) = \sum_I \sum_{j \notin I} \sum_l \partial_l \partial_j f_I \, \varepsilon(j, I) \, e_{(l) K(j, I)}$ .
By fact A, $\varepsilon(j, I) \, e_{(l) K(j, I)} = e_{(l, j) I}$ . For $j \in I$ the sequence $(l, j) I$ has a repeated index, so $e_{(l, j) I} = 0$ . Hence $d(d\omega) = \sum_I \sum_{j, l} \partial_l \partial_j f_I \, e_{(l, j) I}$ .
Fix $I$ . The terms with $j = l$ vanish, because $(l, l) I$ has a repeated index.
For $j \ne l$ , take the two terms for $(j, l)$ and $(l, j)$ . By fact B their sum is $(\partial_l \partial_j f_I - \partial_j \partial_l f_I) \, e_{(l, j) I}$ .
By Schwarz's theorem for the $C^2$ function $f_I$ , $\partial_l \partial_j f_I = \partial_j \partial_l f_I$ . So each such pair contributes $0$ .
By steps 11 to 14, every term of $d(d\omega)$ cancels, and $d(d\omega) = 0$ .

Idea

Applying $d$ twice produces each second derivative $\partial_l \partial_j f$ together with $dx_l \wedge dx_j$ . Swapping $l$ and $j$ leaves the derivative the same for $C^2$ functions but changes the sign of the wedge. So the terms cancel in pairs, and the diagonal terms vanish because $dx_l \wedge dx_l = 0$ .

Reading path

To understand this page from the ground up, read these 385 pages in order. The foundations come first. The path takes the proofs 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 of the decomposition of a permutation into transpositions, 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 that equivalence classes form a partition, Proof of the triangle inequality for norms, Proof that a complex number times its conjugate is the square of its modulus, Proof of the Cauchy–Schwarz inequality, 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 disjoint cycles commute, Proof of the equality of complex numbers, 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 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 laws of exponents in a group, Proof of the limit of a product of sequences, Proof of the linearity of the derivative, Proof that the absolute value is multiplicative, Proof of the squeeze theorem for sequences, Proof of the derivative of a continuous linear map, 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 that a continuous image of a compact set is compact, Proof that compact subsets of a Hausdorff space are closed, Proof that the composition of continuous maps is continuous, Proof of epsilon-delta continuity at a point in metric spaces, 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 Schwarz's theorem for C^2 functions by the mean value theorem, Proof that compactness of a subset can be tested with open sets of the ambient space, Proof that the sign is a homomorphism, 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 Zorn's lemma, Proof that metric spaces are Hausdorff, Proof that the derivative gives the directional derivatives, Proof that differentiable functions are continuous, Proof of the decomposition of a permutation into disjoint cycles, 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 division algorithm, 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 uniqueness of inverses, Proof of the derivative of a map into a product, 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 monotone convergence theorem for sequences, Proof that continuity is equivalent to sequential continuity, Proof of the symmetry of second derivatives by a second difference, Proof that open balls are open, 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 mean value theorem from Rolle's theorem, Proof of continuity via open sets in metric spaces, Proof of the mean value inequality, Proof of the constant multiple rule.

Basis concept
Binary operation definition
Complex numbers object
Complex numbers as pairs of reals concept
Construction of the reals from Cauchy sequences concept
Coordinate space object
Coordinate vector concept
Cycle notation concept
Definition by recursion concept
Divisibility definition
Element definition
Euclidean metric example
Euclidean space object
Extended real number line object
Function concept
Function composition concept
Group concept
Imaginary unit object
Inner product concept
Inner product space concept
Integers object
Limit point of a set definition
Linear isomorphism concept
Linear map concept
Local maximum definition
Mathematical induction concept
Natural numbers object
Norm concept
Norm induced by an inner product concept
Ordered field 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
Identity element definition
Real and imaginary parts definition
Set definition
Fréchet derivative definition
Having a directional derivative definition
Total derivative concept
Function of several variables concept
Vector-valued function concept
Group homomorphism definition
Symmetric group definition
Ring of integers statement
Limit of a function definition
Local minimum definition
Successor function definition
Archimedean ordered field 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
Inner product definition
Linear combination definition
Linear map definition
Linear subspace definition
Norm definition
p-norm on R^n example
Inverse element definition
Complex conjugate definition
Empty set object
Family of sets definition
Ordered pair definition
Differentiable function of several variables concept
Partial derivative concept
Transposition definition
Intersection definition
Limit of a function of several variables concept
Union definition
Parametrised curve concept
Set-builder notation concept
Continuity at a point definition
Derivative definition
Proof of the derivative of a continuous linear map proof
Addition of natural numbers definition
Biconditional definition
Truth table concept
Modus ponens concept
Existential quantifier definition
Universal quantifier definition
Bounded sequence definition
Limit of a sequence definition
Monotone sequence definition
Proof of the Archimedean property in an Archimedean ordered field proof
The rationals form a field statement
The successor function is injective statement
Zero is not a successor statement
Inner product space definition
Linear independence definition
Composition of linear maps definition
Linear isomorphism definition
Linear span definition
Multilinear map definition
Normed vector space definition
Completeness axiom statement
Cartesian product definition
Disjoint sets definition
Proof of the equality of complex numbers proof
Gradient concept
Jacobian matrix concept
Proof of the uniqueness of inverses proof
Derivative of a continuous linear map statement
Partial derivative definition
Continuity in R^n definition
Differentiable function of several variables definition
Union of a family of sets definition
Axiom of extensionality statement
Subset definition
Continuous function definition
Differentiable function definition
Proof of the commutativity of addition by induction proof
Mathematical proof concept
Proof of the derivative as a little-o approximation proof
Propositional logic concept
Archimedean property statement
Uniqueness quantifier definition
Order on the natural numbers definition
Convergent sequence definition
Sequential continuity definition
The rationals form an ordered field statement
Norm induced by an inner product definition
Spanning set definition
Equality of complex numbers statement
Binary relation definition
Higher partial derivatives definition
Uniqueness of inverses statement
Universal set definition
Jacobian matrix definition
Partition of a set definition
Topological space definition
Differentiable on an open set definition
Gradient definition
Derivative as a little-o approximation statement
Proof of the derivative of a map into R^n by components proof
Order on the reals definition
Tautology definition
Commutativity of addition statement
Proof of strong induction from ordinary induction proof
Finite-dimensional vector space definition
Basis definition
Antisymmetric relation definition
Reflexive relation definition
Symmetric relation definition
Transitive relation definition
Iterated derivative definition
Sequential compactness definition
Complement definition
Subspace topology definition
Proof of the laws of exponents in a group proof
Function definition
Interval definition
Logical equivalence definition
Coordinate vector definition
Strong induction statement
Equivalence relation definition
Preorder definition
Codomain definition
Function composition definition
Identity function definition
Injective function definition
Choice function definition
Image of a set definition
Preimage of a set definition
Proof of the epsilon-delta form of the limit of a function proof
Proof of the epsilon-delta form of continuity proof
Laws of exponents in a group statement
Metric space definition
Proof of the well-ordering principle from strong induction proof
Proof of the double negation law by a truth table proof
Partial order definition
Left inverse definition
Right inverse definition
Proof that disjoint cycles commute proof
Surjective function definition
Equivalence class definition
Axiom of choice statement
Epsilon-delta definition of a limit statement
Epsilon-delta definition of continuity statement
Closed ball definition
Convergence of a sequence in a metric space definition
Lipschitz map definition
Open ball definition
Triangle inequality for a metric statement
Double negation law statement
Diameter of a set definition
Well-ordering principle statement
Maximal element definition
Total order definition
Upper bound definition
Bijective function definition
Proof that equivalence classes form a partition proof
Disjoint cycles commute statement
Metric topology definition
Bounded set definition
Proof by contradiction concept
Proof of the division algorithm proof
Proof that every sequence has a monotone subsequence, by peaks proof
Chain definition
Proof that a function is bijective iff it has a two-sided inverse proof
Equivalence classes form a partition statement
Open set definition
Division algorithm statement
Proof that the reals form an ordered field, with the Cauchy sequence construction proof
Every sequence has a monotone subsequence statement
Bijective iff it has a two-sided inverse statement
Hausdorff space definition
Closed set definition
Continuous map of topological spaces definition
Proof of the decomposition of a permutation into disjoint cycles proof
Open cover definition
Proof that open sets are the sets that are neighbourhoods of all their points proof
Equivalent metrics definition
Proof that the derivative gives the directional derivatives proof
Zorn's lemma statement
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
Proof that the composition of continuous maps is continuous proof
Compact space definition
Decomposition of a permutation into disjoint cycles statement
Equivalent norms definition
Open sets characterised by neighbourhoods statement
Derivative gives the directional derivatives statement
Open sets of a metric space statement
Proof that the coordinate map is a linear isomorphism proof
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 the infimum by negation proof
Proof of the existence of square roots of nonnegative reals proof
Complement of an open set is closed statement
Composition of continuous maps is continuous statement
Continuous linear map definition
Compact set definition
Proof of the decomposition of a permutation into transpositions proof
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
Proof of the epsilon-N definition of the limit of a sequence proof
Isomorphism with the coordinate space statement
Every spanning set contains a basis statement
Approximation property of the supremum statement
Existence of the infimum statement
Existence of square roots of nonnegative reals statement
Proof of continuity via closed sets proof
Proof that compactness of a subset can be tested with open sets of the ambient space proof
Decomposition of a permutation into transpositions statement
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 that the reals are Archimedean from the completeness axiom proof
Sign of a permutation definition
Continuity via closed sets statement
Modulus of a complex number definition
Compactness via open covers statement
Proof of the limit of a sum of sequences proof
Proof of the uniqueness of limits proof
Proof of epsilon-delta continuity at a point in metric spaces proof
Proof of continuity via open sets in metric spaces proof
Proof that limits preserve non-strict inequalities proof
Proof of the Lebesgue number lemma by a finite cover of balls proof
Proof of the squeeze theorem for sequences proof
Proof that metric spaces are Hausdorff proof
Proof that differentiable functions are continuous 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 complex number times its conjugate is the square of its modulus proof
Proof that a continuous image of a compact set is compact proof
Proof that the sign is a homomorphism proof
Epsilon-delta continuity in metric spaces statement
Lebesgue number lemma statement
Proof that compact subsets of a metric space are bounded proof
Metric spaces are Hausdorff statement
Proof that compactness and sequential compactness agree in metric spaces proof
Limit of a sum of sequences statement
Proof of the sequence characterisation of uniform continuity proof
Uniqueness of limits statement
Differentiable functions are continuous statement
Limits preserve non-strict inequalities statement
Proof of the triangle inequality for norms proof
Convergent sequences are bounded statement
Squeeze theorem statement
Subsequences of a convergent sequence converge to the same limit statement
Proof that there is a natural number n with 1/n below any positive epsilon proof
Monotone convergence theorem for sequences statement
A complex number times its conjugate statement
Compact subsets of a Hausdorff space are closed statement
Continuous image of a compact set is compact statement
Sign is a homomorphism statement
Compactness and sequential compactness agree in metric spaces statement
Continuity via open sets statement
Compact subsets of a metric space are bounded statement
Proof of the limit of a product of sequences proof
Epsilon-delta uniform continuity statement
There is a natural number n with 1/n below any positive epsilon statement
Proof that a decreasing sequence bounded below converges proof
Triangle inequality for norms statement
Proof that a linear map between normed spaces is continuous if and only if it is bounded 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
Proof of the Cauchy–Schwarz inequality proof
Decreasing sequence bounded below converges statement
Limit of a product of sequences statement
Lipschitz maps are uniformly continuous statement
Continuity is equivalent to sequential continuity statement
Sequential characterization of limits statement
Proof of the Bolzano–Weierstrass theorem by a monotone subsequence proof
Cauchy–Schwarz inequality statement
Bounded linear maps are continuous statement
Proof of the linearity of differentiation proof
Bolzano–Weierstrass theorem statement
Proof of the derivative of a map into a product proof
Proof of the linearity of the derivative proof
Linearity of differentiation statement
Proof of the Heine–Borel theorem by sequential compactness proof
Operator norm definition
Proof of the boundedness theorem by Bolzano–Weierstrass proof
Derivative of a map into a product statement
Linearity of the derivative statement
Proof of the constant multiple rule proof
Heine–Borel theorem statement
Boundedness theorem statement
Constant multiple rule statement
Derivative of a map into R^n by components statement
Proof of the extreme value theorem by the Bolzano–Weierstrass theorem proof
Proof that a continuous real function on a compact set attains its bounds 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 of the equivalence of norms in finite dimension by compactness of the unit sphere proof
Fermat's theorem on stationary points statement
Equivalence of norms in finite dimension statement
Proof of Rolle's theorem proof
Rolle's theorem statement
Proof that linear maps on finite-dimensional normed spaces are continuous proof
Linear maps on finite-dimensional spaces are continuous statement
Proof of the mean value theorem from Rolle's theorem proof
Mean value theorem statement
Proof of the chain rule in several variables proof
Proof that continuous partial derivatives imply differentiability proof
Chain rule in several variables statement
Continuous partial derivatives imply differentiability statement
Proof of the chain rule for a composition with a curve 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
Proof of the mean value inequality proof
Differential form definition
Proof of Schwarz's theorem for C^2 functions by the mean value theorem proof
Parametrised curve definition
Vector field definition
Mean value inequality statement
Wedge product of forms definition
Rectifiable curve definition
Conservative vector field definition
Curl definition
Divergence definition
Proof of the symmetry of second derivatives by a second difference proof
Line integral of a vector field definition
Symmetry of second derivatives statement
Schwarz's theorem for C^2 functions statement
One-form definition
Exterior derivative definition
Proof that the exterior derivative squares to zero proof

Where this leads

1 page builds on this one.

The exterior derivative squares to zero

Tag	`WFNBV54XSE`
URI	`m2://WFNBV54XSE`
Kind	proof
Writing	W0 stub
MSC	58A10
Proves	The exterior derivative squares to zero
Uses	Exterior derivative, Wedge product of forms, Differential form, Schwarz's theorem for C^2 functions, C^k function characterised by continuous derivatives, Sign of a permutation, Sign is a homomorphism, Transposition
Changed	4 November 2026

Proof that the exterior derivative squares to zero

Proof

Idea

Reading path

Where this leads

Further reading

What this page needs for W1

What links here