Il teorema di Darboux

Darboux's theorem This article is about Darboux's theorem in symplectic geometry. For Darboux's theorem related to the intermediate value theorem, see Darboux's theorem (analisi).

Darboux's theorem is a theorem in the mathematical field of differential geometry and more specifically differential forms, partially generalizing the Frobenius integration theorem. It is a foundational result in several fields, the chief among them being symplectic geometry. The theorem is named after Jean Gaston Darboux[1] who established it as the solution of the Pfaff problem.[2] One of the many consequences of the theorem is that any two symplectic manifolds of the same dimension are locally symplectomorphic to one another. Questo è, every 2n-dimensional symplectic manifold can be made to look locally like the linear symplectic space Cn with its canonical symplectic form. There is also an analogous consequence of the theorem as applied to contact geometry.

Contenuti 1 Statement and first consequences 2 Comparison with Riemannian geometry 3 Guarda anche 4 Appunti 5 Riferimenti 6 External links Statement and first consequences The precise statement is as follows.[3] Supporre che {stile di visualizzazione theta } is a differential 1-form on an n dimensional manifold, tale che {displaystyle matematica {d} teta } has constant rank p. Se {displaystyle theta wedge left(matematica {d} theta right)^{p}=0} ovunque, then there is a local system of coordinates {stile di visualizzazione x_{1},ldot ,X_{n-p},si_{1},ldot ,si_{p}} in quale {displaystyle theta =x_{1},matematica {d} si_{1}+ldots +x_{p},matematica {d} si_{p}} .

Se, d'altro canto, {displaystyle theta wedge left(matematica {d} theta right)^{p}neq 0} ovunque, then there is a local system of coordinates ' {stile di visualizzazione x_{1},ldot ,X_{n-p},si_{1},ldot ,si_{p}} in quale {displaystyle theta =x_{1},matematica {d} si_{1}+ldots +x_{p},matematica {d} si_{p}+matematica {d} X_{p+1}} .

Nota che se {displaystyle theta wedge left(matematica {d} theta right)^{p}neq 0} everywhere and {displaystyle n=2p+1} poi {stile di visualizzazione theta } is a contact form.

In particolare, supporre che {stile di visualizzazione omega } is a symplectic 2-form on an n=2m dimensional manifold M. In a neighborhood of each point p of M, by the Poincaré lemma, there is a 1-form {stile di visualizzazione theta } insieme a {displaystyle matematica {d} theta =omega } . Inoltre, {stile di visualizzazione theta } satisfies the first set of hypotheses in Darboux's theorem, and so locally there is a coordinate chart U near p in which {displaystyle theta =x_{1},matematica {d} si_{1}+ldots +x_{m},matematica {d} si_{m}} .

Taking an exterior derivative now shows {displaystyle omega =mathrm {d} theta =mathrm {d} X_{1}wedge mathrm {d} si_{1}+ldots +mathrm {d} X_{m}wedge mathrm {d} si_{m}} The chart U is said to be a Darboux chart around p.[4] The manifold M can be covered by such charts.

To state this differently, identify {displaystyle mathbb {R} ^{2m}} insieme a {displaystyle mathbb {C} ^{m}} by letting {stile di visualizzazione z_{j}=x_{j}+{textit {io}},si_{j}} . Se {displaystyle varphi colon Uto mathbb {C} ^{n}} is a Darboux chart, poi {stile di visualizzazione omega } is the pullback of the standard symplectic form {stile di visualizzazione omega _{0}} Su {displaystyle mathbb {C} ^{n}} : {displaystyle omega =phi ^{*}omega _{0}.,} Comparison with Riemannian geometry This result implies that there are no local invariants in symplectic geometry: a Darboux basis can always be taken, valid near any given point. This is in marked contrast to the situation in Riemannian geometry where the curvature is a local invariant, an obstruction to the metric being locally a sum of squares of coordinate differentials.

The difference is that Darboux's theorem states that ω can be made to take the standard form in an entire neighborhood around p. Nella geometria riemanniana, the metric can always be made to take the standard form at any given point, but not always in a neighborhood around that point.

See also Carathéodory–Jacobi–Lie theorem, a generalization of this theorem. Symplectic basis Notes ^ Darboux (1882). ^ Pfaff (1814–1815). ^ Sternberg (1964) p. 140–141. ^ Cfr. with McDuff and Salamon (1998) p. 96. References Darboux, Gaston (1882). "Sur le problème de Pfaff". Toro. Sci. Matematica. 6: 14–36, 49–68. Pfaff, Johann Friedrich (1814–1815). "Methodus generalis, aequationes differentiarum partialium nec non aequationes differentiales vulgates, ultrasque primi ordinis, inter quotcunque variables, complete integrandi". Abhandlungen der Königlichen Akademie der Wissenschaften in Berlin: 76–136. Sternberg, Shlomo (1964). Lectures on Differential Geometry. Sala dell'Apprendista. McDuff, D.; Salamon, D. (1998). Introduction to Symplectic Topology. la stampa dell'università di Oxford. ISBN 0-19-850451-9. link esterno "Proof of Darboux's Theorem". PlanetMath. G. Darboux, "On the Pfaff Problem," transl. by D. H. Delphenich G. Darboux, "On the Pfaff Problem (cont.)," transl. by D. H. Delphenich Categories: Differential systemsSymplectic geometryCoordinate systems in differential geometryTheorems in differential geometryMathematical physics

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