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Symplectomorphism

A symplectomorphism is a \phi: (M_1, \omega_1) \to (M_2, \omega_2) between two manifolds that preserves the forms, meaning \phi^*\omega_2 = \omega_1. This preservation ensures that the nondegenerate, closed 2-form structure defining the is maintained under the map. In , symplectomorphisms serve as the structure-preserving maps between manifolds, analogous to isometries in , and they form the symplectomorphism group \mathrm{Symp}(M, \omega) on a given manifold. A key property is that every is locally symplectomorphic to the standard space (\mathbb{R}^{2n}, \omega_0 = \sum_{i=1}^n dx_i \wedge dy_i) by the Darboux theorem, highlighting the uniformity of local structure despite global variations. The graphs of symplectomorphisms are submanifolds, which are maximal isotropic subspaces with respect to the form, underscoring their role in studying intersections and fixed points. On compact manifolds with vanishing first , symplectomorphisms close to the identity possess at least two fixed points, as per extensions of the Arnold conjecture. Symplectomorphisms are fundamental in , where they correspond to canonical transformations that preserve the structure and Poisson brackets \{f, g\} = \omega(X_f, X_g). The time evolution of a generates a one-parameter group of symplectomorphisms via the flow of the X_H, defined by \omega(X_H, \cdot) = -dH, ensuring conservation of the volume and . In broader applications, they facilitate symplectic reduction and moment maps for symmetries, enabling the analysis of conserved quantities and equivariant structures in dynamical systems.

Definition and Basics

Formal Definition

A symplectic manifold is an even-dimensional smooth manifold M equipped with a symplectic form \omega, which is a closed non-degenerate 2-form on M. The closedness condition requires that the exterior derivative vanishes, d\omega = 0, ensuring that \omega defines a presymplectic structure that is compatible with the manifold's topology. Non-degeneracy means that for every point p \in M, the map \tilde{\omega}_p: T_p M \to T_p^* M given by \tilde{\omega}_p(v)(u) = \omega_p(v, u) is a linear isomorphism, implying that \omega_p pairs tangent vectors with cotangent vectors bijectively. A is a f: (M, \omega) \to (N, \omega') between two manifolds that preserves the structure, satisfying f^* \omega' = \omega. The operation f^* \omega' is defined pointwise by (f^* \omega')_p(u, v) = \omega'_{f(p)}(df_p(u), df_p(v)) for p \in M and tangent vectors u, v \in T_p M, which ensures that the form on N is transported back to match \omega on M. In local Darboux coordinates (q_1, \dots, q_n, p_1, \dots, p_n) on M and similar coordinates on N, where \omega = \sum_{i=1}^n dq_i \wedge dp_i and \omega' = \sum_{i=1}^n dq_i' \wedge dp_i', the condition f^* \omega' = \omega translates to the matrix J = df_p satisfying J^T \Omega J = \Omega, with \Omega = \begin{pmatrix} 0 & I_n \\ -I_n & 0 \end{pmatrix} the standard matrix. Symplectomorphisms are distinguished as or depending on their domain: a symplectomorphism is defined on the entire manifolds M and N, while a symplectomorphism is a between open subsets U \subset M and V \subset N satisfying the condition on those sets, though it may not extend to the whole manifolds due to topological obstructions.

Examples

In , symplectomorphisms appear as canonical transformations on the \mathbb{R}^{2n}, endowed with the standard form \omega = \sum_{i=1}^n dq_i \wedge dp_i. These transformations map old coordinates (q_i, p_i) to new ones (Q_i, P_i) while preserving the of Hamilton's equations, meaning the transformed retains the same form. Equivalently, a \phi: \mathbb{R}^{2n} \to \mathbb{R}^{2n} is canonical if it satisfies \phi^* \omega = \omega, ensuring the is . Linear symplectomorphisms on \mathbb{R}^{2n} form the symplectic group Sp(2n, \mathbb{R}), consisting of all $2n \times 2n invertible real matrices A that preserve the standard symplectic form, satisfying A^T J A = J where J = \begin{pmatrix} 0 & I_n \\ -I_n & 0 \end{pmatrix}. These matrices represent linear changes of coordinates in phase space that maintain the symplectic structure, such as rotations in the (q_i, p_i)-planes combined with appropriate adjustments in conjugate variables. For instance, in n=1, elements of Sp(2, \mathbb{R}) include matrices like \begin{pmatrix} a & b \\ c & d \end{pmatrix} with ad - bc = 1, preserving areas in the (q, p)-plane. In two dimensions, symplectomorphisms simplify to area-preserving s. On \mathbb{R}^2 with \omega = dq \wedge dp, any preserving this form locally preserves areas, as \omega defines the standard area measure. Similarly, on the two-dimensional T^2, equipped with a compatible area form, symplectomorphisms are orientation-preserving s that conserve the total area, playing a key role in studies of dynamical systems like twist maps. Examples include the on T^2, which models chirikov's in while preserving the symplectic area. A concrete example of a nonlinear symplectomorphism on \mathbb{R}^{2n} arises from coordinate changes that preserve the symplectic structure, such as the transformation (q_i, p_i) \mapsto (q_i + f_i(p), p_i) for smooth functions f_i: \mathbb{R}^n \to \mathbb{R}. For n=1, the map (q, p) \mapsto (q + f(p), p) satisfies \phi^* \omega = dq \wedge dp + df \wedge dp = dq \wedge dp, since df \wedge dp = f'(p) \, dp \wedge dp = 0, thus preserving the form. This type of shear transformation is canonical and can simplify Hamiltonians, for example, in action-angle variables.

Dynamical Properties

Hamiltonian Flows

In symplectic geometry, the Hamiltonian vector field X_H associated to a smooth function H: (M, \omega) \to \mathbb{R} on a symplectic manifold (M, \omega) is defined by the equation \omega(X_H, \cdot) = -dH. This vector field encodes the dynamics of the classical mechanical system governed by the Hamiltonian H, where \omega is the symplectic form. The integral curves of X_H generate a one-parameter group of diffeomorphisms \phi_t: M \to M, known as the Hamiltonian flow, satisfying \frac{d}{dt} \phi_t(p) = X_H(\phi_t(p)) with \phi_0 = \mathrm{id}. This flow preserves the symplectic structure, meaning \phi_t^* \omega = \omega for all t, establishing that each \phi_t is a symplectomorphism. The preservation arises because the Lie derivative of the symplectic form along X_H vanishes: \mathcal{L}_{X_H} \omega = 0. To see this, note that \mathcal{L}_{X_H} \omega = d(i_{X_H} \omega) + i_{X_H} d\omega = d(-dH) + 0 = 0, since d\omega = 0 by the closedness of \omega. Additionally, the conserves levels, satisfying H \circ \phi_t = H for all t. This follows from the fact that along trajectories, \frac{d}{dt} (H \circ \phi_t) = dH(X_H) = -[\omega](/page/Omega)(X_H, X_H) = 0, as [\omega](/page/Omega) is skew-symmetric. A key consequence is , which states that the Hamiltonian preserves the \frac{\omega^n}{n!} on the $2n-dimensional manifold M, where n is the dimension of the underlying real . This ensures that volumes remain invariant under time evolution, reflecting the incompressibility of Hamiltonian dynamics.

Group of Symplectomorphisms

The group of symplectomorphisms of a (M, \omega), denoted \mathrm{Symp}(M, \omega), consists of all diffeomorphisms \phi: M \to M satisfying \phi^*\omega = \omega. This group is equipped with the C^\infty-topology and forms an infinite-dimensional pseudogroup. Within \mathrm{Symp}(M, \omega), the \mathrm{Ham}(M, \omega) comprises the symplectomorphisms, which are the time-1 maps of flows generated by Hamiltonian vector fields. This is and path-connected. For compact connected symplectic manifolds, Banyaga's theorem asserts that \mathrm{Ham}(M, \omega) is a , meaning it has no nontrivial normal subgroups. The Hofer metric on \mathrm{Ham}(M, \omega) is defined as the infimum over all Hamiltonians generating a path from the to a given element, yielding a complete bi-invariant that induces a Finsler geometry on the group. Not all elements of \mathrm{Symp}(M, \omega) belong to \mathrm{Ham}(M, \omega), particularly on non-compact manifolds where the homomorphism detects non-Hamiltonian symplectomorphisms; for instance, on the S^1 \times \mathbb{R} with the standard symplectic form, certain area-preserving maps isotopic to the identity through symplectomorphisms are not .

Geometric Comparisons

With Riemannian Geometry

In Riemannian geometry, an isometry is a diffeomorphism \phi: (M, g) \to (M', g') that preserves the metric tensor, satisfying g'(\mathrm{d}\phi(X), \mathrm{d}\phi(Y)) = g(X, Y) for all vector fields X, Y. The group of isometries \mathrm{Isom}(M, g) of a compact Riemannian manifold is a finite-dimensional Lie group, typically of dimension at most \frac{1}{2} \dim(M) (\dim(M) + 1). In contrast, symplectomorphisms preserve only the symplectic form \omega, satisfying \phi^* \omega' = \omega, and the group \mathrm{Symp}(M, \omega) of symplectomorphisms of a symplectic manifold is an infinite-dimensional Fréchet Lie group, exhibiting significantly less rigidity than the isometry group. A representative example illustrates this difference on the 2-sphere S^2. Equipped with the standard round metric, the is the O(3), a compact 3-dimensional consisting of rotations and reflections. However, with the standard area form as symplectic structure, the symplectomorphism group is larger, comprising all area-preserving diffeomorphisms, and is infinite-dimensional. These structural disparities have profound implications for classification. Riemannian manifolds are locally determined by their curvature tensor, allowing distinction via local invariants like . Symplectic manifolds, however, admit Darboux coordinates locally, with no nontrivial local invariants beyond the dimension, underscoring the greater flexibility in .

Rigidity and Local Structure

One of the fundamental results in is the Darboux theorem, which asserts that every (M, \omega) of dimension $2n is locally symplectomorphic to the standard space (\mathbb{R}^{2n}, \omega_0), where \omega_0 = \sum_{i=1}^n dq_i \wedge dp_i. This means that around any point in M, there exist local coordinates (q_1, \dots, q_n, p_1, \dots, p_n) such that the symplectic form \omega takes the \omega_0 in these coordinates. The theorem highlights the local flexibility of structures, as it implies that possess no local invariants analogous to in . A related local normalization result is the Moser theorem, which addresses the stability of symplectic forms under deformations. Specifically, if two symplectic forms \omega_0 and \omega_1 on a compact manifold M are isotopic through a smooth path \omega_t (with t \in [0,1]) such that [\omega_t] = [\omega_0] in the (i.e., they lie in the same class), then there exists a \phi: M \to M isotopic to the identity such that \phi^* \omega_1 = \omega_0. This theorem, often proved using Moser's trick of solving a certain equation for vector fields, demonstrates that symplectic forms within the same cohomology class are equivalent up to symplectomorphism on compact manifolds. These local theorems have profound implications for the classification of symplectic manifolds. Unlike Riemannian manifolds, where local invariants like provide obstructions to , symplectic manifolds lack such local invariants due to the Darboux theorem, making local classification trivial. However, global topological features, such as the or , play a crucial role in distinguishing symplectic structures, as the Moser theorem preserves classes but does not address global embedding or topological obstructions. Despite this local flexibility, symplectic geometry exhibits remarkable global rigidity phenomena, as exemplified by the Gromov width, a symplectic capacity that measures the largest standard ball embeddable into a while respecting the symplectic structure. The Gromov nonsqueezing theorem establishes that the Gromov width of a provides a rigid obstruction to embeddings, preventing "squeezing" of higher-dimensional balls into lower-dimensional cylinders symplectically. This rigidity contrasts sharply with the local triviality from Darboux and Moser theorems, underscoring how symplectic invariants emerge globally through techniques like pseudoholomorphic curves.

Advanced Topics

Quantizations

Quantization procedures in provide a bridge between on symplectic manifolds and on s, where symplectomorphisms play a central role by inducing s that preserve the quantum structure. In this framework, a classical (M, \omega) is quantized to a \mathcal{H}, and a symplectomorphism \phi: M \to M lifts to a U_\phi: \mathcal{H} \to \mathcal{H} such that the quantization map intertwines the classical and quantum actions, ensuring that quantum observables evolve according to the classical symplectic dynamics. This correspondence is foundational in , as developed by Kostant and Souriau, where the symplectic form \omega dictates the commutation relations in the quantum algebra. Prequantization is the initial step, associating to the symplectic manifold (M, \omega) a complex line bundle L \to M equipped with a Hermitian connection whose curvature form equals \omega / \hbar, where \hbar is the reduced Planck's constant. Symplectomorphisms on M that preserve the cohomology class of \omega lift to automorphisms of the prequantum bundle L, preserving the connection and thus the parallel transport, which corresponds to the classical flow in the quantum setting. For Hamiltonian symplectomorphisms generated by a function H, the lift is explicitly given by multiplication by the phase factor \exp(i H / \hbar) on sections of L, ensuring a unitary representation. However, the prequantum Hilbert space L^2(M, L) is typically infinite-dimensional and overcomplete, necessitating further refinement. Geometric quantization refines prequantization by incorporating a choice of polarization—a maximal positive Lagrangian subbundle of the complexified tangent bundle—and half-forms to correct for the transformation properties under symplectomorphisms. The quantum consists of square-integrable sections of L \otimes K^{1/2}, where K is the , that are holomorphic along the ; symplectomorphisms act on these sections via the , which ensures unitarity up to a phase determined by the Maslov index. This arises from the action on half-densities and captures the quantum evolution, with observables quantized as operators on the polarized sections. A concrete example occurs on the standard symplectic space \mathbb{R}^{2n} with the canonical form \omega = \sum dq_i \wedge dp_i, where the group of linear symplectomorphisms is \mathrm{Sp}(2n, \mathbb{R}), and its double cover is the \mathrm{Mp}(2n, \mathbb{R}). The metaplectic representation provides unitary operators on the quantum L^2(\mathbb{R}^n), faithfully realizing \mathrm{Mp}(2n, \mathbb{R}) and thus \mathrm{Sp}(2n, \mathbb{R}) projectively; for the , this action rotates the coordinates while preserving the levels, illustrating how classical symplectic transformations quantize to ladder operators. Challenges arise because not all symplectomorphisms quantize to unitary operators without anomalies; topological obstructions, such as the non-vanishing of the Maslov class or the failure to to the prequantum bundle, prevent a consistent unitary for general symplectomorphisms, particularly those not isotopic to the on non-simply connected manifolds. These anomalies manifest as phase inconsistencies in the representation, requiring the double cover to resolve them in linear cases but leading to projective rather than true unitary representations in general.

Arnold Conjecture

The Arnold conjecture, proposed by in 1965, asserts that for a non-degenerate symplectomorphism \phi on a compact (M, \omega) of dimension $2n, the number of fixed points of \phi is at least the sum of the Betti numbers of M, that is, \# \operatorname{Fix}(\phi) \geq \sum_{i=0}^{2n} b_i(M). This lower bound exceeds the \chi(M) in general and provides a sharp homological estimate for the minimal number of fixed points. The conjecture applies specifically to the subgroup of the symplectomorphism group, where \phi is the time-1 map of a generated by some smooth time-dependent H: S^1 \times M \to \mathbb{R}. The motivation for the conjecture draws from , viewing diffeomorphisms as analogous to gradient flows of Morse functions on M. Fixed points of \phi correspond to critical points of the symplectic action functional on the loop space of M, whose Morse inequalities would imply the desired bound if a suitable infinite-dimensional could be developed. Partial results toward the conjecture include weaker estimates from the Lusternik–Schnirelmann category: for a symplectomorphism homotopic to the identity, the number of fixed points is at least the LS category of M plus one, which is bounded above by \sum b_i(M) but often strictly smaller. Full proofs exist in low dimensions, such as for surfaces (where the bound is \sum b_i = 2 + 2g for g) and tori, using variational methods or holomorphic curve techniques. A major breakthrough came with the introduction of Hamiltonian Floer homology by Andreas Floer in 1986, which defines a generated by non-degenerate 1-periodic orbits (fixed points of \phi) and differentials via moduli spaces of pseudoholomorphic cylinders; this yields an isomorphism with the ordinary cohomology of M over suitable Novikov rings, proving the conjecture for semi-positive (including ) symplectic manifolds. The theory was later extended to all closed symplectic manifolds without positivity assumptions, using virtual techniques to handle bubbling issues. The conjecture has profound implications for symplectic topology, establishing minimal numbers of periodic orbits for generic Hamiltonians and, through symplectization, linking to the existence of closed Reeb orbits on associated contact manifolds, as in analogs of the Weinstein conjecture. As of 2025, the conjecture is proven in many cases, including all monotone symplectic manifolds via and its refinements, and fully for general closed symplectic manifolds over the rationals using A_\infty-structures and Gromov–Witten invariants, as independently shown by Fukaya–Ono and . Extensions to relative or singular settings continue to leverage these tools, though the conjecture remains open over the integers in full generality.

References

  1. [1]
    [PDF] Lectures on Symplectic Geometry
    Definition 1.4 A symplectomorphism ϕ between symplectic vector spaces (V, Ω) and (V 0, Ω0) is a linear isomorphism ϕ : V. ≃. → V 0 such that ϕ∗Ω0 = Ω. (By.
  2. [2]
    [PDF] Hamiltonian Mechanics and Symplectic Geometry
    preserving the symplectic structure (f∗(ω) = ω) is called a symplectomorphism, and corresponds to the physicist's notion of a canonical transformation of phase.
  3. [3]
    Symplectic maps - Scholarpedia
    Mar 13, 2010 · A symplectic map is a diffeomorphism that preserves a symplectic structure. The simplest example of symplectic map is a map F:{\mathbb R}^2 ...
  4. [4]
    [PDF] C:\Downloaded_files\Arnold V I Mathematical Methods Of Classical ...
    In this book we construct the mathematical apparatus of classical mechanics from the very beginning; thus, the reader is not assumed to have any previous ...
  5. [5]
    [PDF] PHY411 Lecture notes – Canonical Transformations
    Sep 27, 2023 · If the coordinate transformation is canonical and the Poisson brackets are satisfied, then the transformation is symplectic. Take a look again ...Missing: source | Show results with:source
  6. [6]
    [PDF] arXiv:2101.03534v2 [math.SG] 6 Aug 2024
    Aug 6, 2024 · The Hamiltonian vector field XF of a smooth function F : M → R on a symplectic manifold (M,ω) is defined by XF ⌟ω = dF; the Hamiltonian flow of ...
  7. [7]
    [PDF] Towards exact symplectic integrators from Liouvillian forms - arXiv
    Nov 2, 2020 · The flow of a Hamiltonian vector field preserves the symplectic form on M which is characterized by the condition. LXH ω = 0, where LXH ω is the ...
  8. [8]
    [PDF] arXiv:math/0110134v1 [math.SP] 12 Oct 2001
    Hamiltonian flow preserves the Hamiltonian, the canonical 2-form and canonical measure on T∗M (which are defined as dx∧dξ and dx dξ in every local ...
  9. [9]
    [PDF] Time irreversibility from symplectic non-squeezing - arXiv
    Dec 9, 2017 · which is Liouville's theorem on the preservation of the symplectic volume under Hamiltonian flows. We see that the invariance of ω under ...
  10. [10]
    [PDF] A survey of the topological properties of symplectomorphism groups
    Apr 19, 2004 · The symplectomorphism group Symp(M,ω) consists of all diffeomorphisms φ : M → M such that. φ∗(ω) = ω, and is equipped with the C∞-topology, the ...
  11. [11]
    [PDF] Symplectomorphism Groups and Almost Complex Structures - arXiv
    Dec 26, 2004 · Thus the group Symp(M,ω) of all symplectomorphisms of (M,ω) is always infinite dimensional. ... simple definition of these maps. Observe ...
  12. [12]
    [PDF] The Flux homomorphism - MIT Mathematics
    The set Ham(M,ω) ⊆ Symp(M,ω) is a normal and path-connected subgroup. Proof. We can check that if {ψt}, {φt} are Hamiltonian isotopies generated by Ht and Gt, ...
  13. [13]
    [PDF] Lectures on groups of symplectomorphisms - arXiv
    Moser's theorem implies that the groups Symp(M,ω) and Ham(M,ω) depend only on the diffeomorphism class of the form ω. In particular, they do not change their ...
  14. [14]
    Hofer'sL ∞-geometry: energy and stability of Hamiltonian flows, part II
    In this paper we first show that the necessary condition introduced in our previous paper is also a sufficient condition for a path to be a geodesic in the.Missing: original | Show results with:original
  15. [15]
    [PDF] Symplectic Geometry
    The classification of symplectic manifolds up to symplectomorphism is an open prob- lem in symplectic geometry. However, the local classification is taken care ...Missing: textbook | Show results with:textbook
  16. [16]
    2)-spheres and the symplectomorphism group of small rational 4 ...
    Feb 2, 2020 · Let (X, ω) be a symplectic rational surface. We study the space of tamed almost complex structures 7ω using a fine decomposition via smooth.
  17. [17]
    [PDF] arXiv:1405.1460v1 [math.MG] 6 May 2014
    May 6, 2014 · In this paper simple presentations of the isometry groups of Euclidean plane,. 2-sphere, the real projective plane and groups SO(3) and O(n) are ...
  18. [18]
    [PDF] Lectures on the Geometry of Quantization - Berkeley Math
    These lectures introduce microlocal analysis and symplectic geometry, focusing on the transition between classical and quantum mechanics, starting with the ...
  19. [19]
    [PDF] arXiv:math/0602168v1 [math.SG] 8 Feb 2006
    Section 8 is devoted to the action of the prequantization bundle automorphism group on the quantum space bundle. 2. Preliminaries. Let (E,ω) be a symplectic ...
  20. [20]
    [PDF] Lecture Notes on Geometric Quantization
    Aug 27, 2025 · Objective. These Lecture Notes were intended to support the course ”Geometric Quantization” held in the winter term 2021/2022 at the LMU ...
  21. [21]
    [PDF] The symplectic group and the oscillator representation
    As in the case of the rotation group, one can get a true representation by going to a double cover of Sp(2n, R), which we'll denote Mp(2n, R) and call the “ ...
  22. [22]
    [PDF] THE SYMPLECTIC AND METAPLECTIC GROUPS IN QUANTUM ...
    The example of the quantum harmonic oscillator has illustrated how the double covering of the symplectic rotation sub-group by the metaplectic group is ...
  23. [23]
    A Lefschetz fixed point formula for symplectomorphisms
    Following the geometric quantization scheme, the associated quantum spaces are the spaces of holomorphic sections of the tensor powers of the prequantum bundle.<|control11|><|separator|>
  24. [24]
  25. [25]
  26. [26]
    [PDF] Fixed points of symplectic tranformations
    May 20, 2015 · We prove below Arnold's fixed point conjecture for the 2-torus, but we will only prove existence of 1 fixed point. A slightly more precise ...
  27. [27]
    Proof of the Arnold conjecture for surfaces and generalizations to ...
    March 1986 Proof of the Arnold conjecture for surfaces and generalizations to certain Kähler manifolds. Andreas Floer · DOWNLOAD PDF + SAVE TO MY LIBRARY.Missing: original URL
  28. [28]
    floer homology and arnold conjecture - Project Euclid
    As a consequence of this, we extended Floer (co-)homology to all symplectic manifolds without any positivity assumption and proved Arnold conjec- ture in ...
  29. [29]
    The Conley Conjecture and Beyond | Arnold Mathematical Journal
    Jun 4, 2015 · Furthermore, all such Hamiltonian diffeomorphisms are non-degenerate, and the number of fixed points is exactly equal to the sum of Betti ...
  30. [30]
    [PDF] arnold conjecture and gromov-witten invariant - UC Berkeley math
    One of the main results of this paper proves a version of Arnold conjecture on ... Fukaya and K. Ono. Let pr: EX be an orbibundle and so e C°(X; E). We consider ...
  31. [31]
    [2212.01344] The Arnold conjecture for singular symplectic manifolds
    Dec 2, 2022 · In this article, we study the Hamiltonian dynamics on singular symplectic manifolds and prove the Arnold conjecture for a large class of b^m-symplectic ...