Riesz–Thorin theorem

In mathematics, certain systems of partial differential equations are usefully formulated, from the point of view of their underlying geometric and algebraic structure, in terms of a system of differential forms. The idea is to take advantage of the way a differential form restricts to a submanifold, and the fact that this restriction is compatible with the exterior derivative. This is one possible approach to certain over-determined systems, for example. A Pfaffian system is one specified by 1-forms alone, but the theory includes other types of example of differential system.

Given a collection of differential 1-forms α_i, i=1,2, ..., k on an n-dimensional manifold M, an integral manifold is a submanifold whose tangent space at every point p ∈ M is annihilated by each α_i.

A maximal integral manifold is a submanifold

$i:N\subset M$

such that the kernel of the restriction map on forms

$i^{*}:{\mathrm{\Omega }}_p^1(M)\to {\mathrm{\Omega }}_p^1(N)$

is spanned by the α_i at every point p of N. If in addition the α_i are linearly independent, then N is (n − k)-dimensional. Note that i: N ⊂ M need not be an embedded submanifold.

A Pfaffian system is said to be completely integrable if N admits a foliation by maximal integral manifolds. (Note that the foliation need not be regular; i.e. the leaves of the foliation might not be embedded submanifolds.)

An integrability condition is a condition on the α_i to guarantee that there will be integral submanifolds of sufficiently high dimension.

Necessary and sufficient conditions

The necessary and sufficient conditions for complete integrability of a Pfaffian system are given by the Frobenius theorem. One version states that if the ideal $\mathcal{ℐ}$ algebraically generated by the collection of α_i inside the ring Ω(M) is differentially closed, in other words

$d\mathcal{ℐ}\subset \mathcal{ℐ},$

then the system admits a foliation by maximal integral manifolds. (The converse is obvious from the definitions.)

Example of a non-integrable system

Not every Pfaffian system is completely integrable in the Frobenius sense. For example, consider the following one-form on R³ - (0,0,0)

$\theta =xdy+ydz+zdx.$

If dθ were in the ideal generated by θ we would have, by the skewness of the wedge product

$\theta \wedge d\theta =0.$

But a direct calculation gives

$\theta \wedge d\theta =(x+y+z)dx\wedge dy\wedge dz$

which is a nonzero multiple of the standard volume form on R³. Therefore, there are no two-dimensional leaves, and the system is not completely integrable.

On the other hand, the curve defined by

$x=t,y=c,z=e^{-\frac{t}{c}},t>0$

is easily verified to be a solution (i.e. an integral curve) for the above Pfaffian system for any nonzero constant c.

Examples of applications

In Riemannian geometry, we may consider the problem of finding an orthogonal coframe θⁱ, i.e., a collection of 1-forms forming a basis of the cotangent space at every point with $⟨{\theta }^i,{\theta }^j⟩={\delta }^{ij}$ which are closed (dθⁱ = 0, i=1,2, ..., n). By the Poincaré lemma, the θⁱ locally will have the form dxⁱ for some functions xⁱ on the manifold, and thus provide an isometry of an open subset of M with an open subset of Rⁿ. Such a manifold is called locally flat.

This problem reduces to a question on the coframe bundle of M. Suppose we had such a closed coframe

$\mathrm{\Theta }=({\theta }^1,\dots ,{\theta }^n)$.

If we had another coframe $\mathrm{\Phi }=({\varphi }^1,\dots ,{\varphi }^n)$, then the two coframes would be related by an orthogonal transformation

$\mathrm{\Phi }=M\mathrm{\Theta }$

If the connection 1-form is ω, then we have

$d\mathrm{\Phi }=\omega \wedge \mathrm{\Phi }$

On the other hand,

$\begin{array}{rl}d\mathrm{\Phi }& =(dM)\wedge \mathrm{\Theta }+M\wedge d\mathrm{\Theta }\\ & =(dM)\wedge \mathrm{\Theta }\\ & =(dM)M^{-1}\wedge \mathrm{\Phi }.\end{array}$

But $\omega =(dM)M^{-1}$ is the Maurer–Cartan form for the orthogonal group. Therefore it obeys the structural equation $d\omega +\omega \wedge \omega =0,$ and this is just the curvature of M: $\mathrm{\Omega }=d\omega +\omega \wedge \omega =0.$ After an application of the Frobenius theorem, one concludes that a manifold M is locally flat if and only if its curvature vanishes.

Generalizations

Many generalizations exist to integrability conditions on differential systems which are not necessarily generated by one-forms. The most famous of these are the Cartan-Kähler theorem, which only works for real analytic differential systems, and the Cartan–Kuranishi prolongation theorem. See Further reading for details.

Further reading

• Bryant, Chern, Gardner, Goldschmidt, Griffiths, Exterior Differential Systems, Mathematical Sciences Research Institute Publications, Springer-Verlag, ISBN 0-387-97411-3
• Olver, P., Equivalence, Invariants, and Symmetry, Cambridge, ISBN 0-521-47811-1
• Ivey, T., Landsberg, J.M., Cartan for Beginners: Differential Geometry via Moving Frames and Exterior Differential Systems, American Mathematical Society, ISBN 0-8218-3375-8