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Symplectic manifold

A symplectic manifold is a pair (M, \omega) consisting of a smooth manifold M of even dimension $2n and a closed nondegenerate differential 2-form \omega, called the symplectic form, which satisfies d\omega = 0 and induces an isomorphism between the tangent space and cotangent space at every point.^[1]^[2] The nondegeneracy condition ensures that \omega^n / n! defines a natural volume form on M, providing a canonical orientation and enabling the study of Hamiltonian dynamics via vector fields X_H satisfying \iota_{X_H} \omega = -dH for smooth functions H: M \to \mathbb{R}.^[2]^[1] Symplectic manifolds form the foundational structure of symplectic geometry, a field that generalizes classical mechanics to infinite-dimensional settings and connects differential geometry with topology and physics.^[2] By the Darboux theorem, every symplectic manifold is locally symplectomorphic to the standard symplectic \mathbb{R}^{2n} with form \sum_{i=1}^n dx_i \wedge dy_i, highlighting their uniform local structure despite global complexity.^[2] Key properties include the preservation of \omega under symplectomorphisms, which are diffeomorphisms pulling back \omega to itself, and the existence of moment maps for Hamiltonian group actions, linking symmetries to conserved quantities via Noether's theorem.^[2]^[1] Prominent examples include the cotangent bundle T^*Q of any manifold Q, equipped with the canonical symplectic form \sum dq_i \wedge dp_i, which models phase spaces in Lagrangian and Hamiltonian mechanics.^[1]^[2] Other instances are the 2-sphere S^2 with the form \omega_p(u,v) = \langle p, u \times v \rangle, Kähler manifolds like complex projective space \mathbb{CP}^n with the Fubini-Study metric, and toric varieties arising from torus actions.^[2] These structures underpin applications in classical and quantum physics, such as rigid body motion and gauge theories, and in mathematics, including symplectic reduction via the Marsden-Weinstein theorem, which quotients out symmetries to yield new symplectic manifolds.^[2]^[1] Historically, symplectic manifolds emerged in the 19th century through the study of Hamiltonian systems in classical mechanics, with the term "symplectic" coined by Hermann Weyl in the early 20th century for the associated linear group.^[2] Modern developments accelerated in the 1960s with Vladimir Arnold's conjectures on fixed points of symplectomorphisms, resolved using Floer homology, and Alan Weinstein's foundational work on Lagrangian submanifolds and reduction.^[2] Further advances, including Mikhail Gromov's introduction of pseudoholomorphic curves in 1985 and convexity theorems by Duistermaat-Heckman and Atiyah-Guillemin-Sternberg, have deepened connections to algebraic geometry and topology.^[2]

Introduction and Motivation

Historical Development

The foundations of symplectic geometry trace back to classical mechanics in the early 19th century. In 1809, Siméon Denis Poisson introduced the Poisson bracket as a tool for analyzing variations in mechanical systems, providing a algebraic structure on phase space coordinates that later proved essential for understanding Hamiltonian dynamics.^[3] This bracket captured the interdependence of position and momentum variables, laying groundwork for the geometric interpretation of phase spaces. Shortly thereafter, in 1834, William Rowan Hamilton developed his principal function, which formalized the variational principle for mechanical systems and led to Hamilton's equations of motion, emphasizing the role of phase space as a geometric arena for dynamics.^[4] The mathematical formalization of these ideas accelerated in the 20th century through connections with differential geometry and group theory. In 1939, Hermann Weyl coined the term "symplectic" for the symplectic group, drawing from Greek roots to distinguish it from complex structures while highlighting its role in preserving volume in phase space transformations.^[5] Post-World War II, the French school advanced the concept significantly: in 1948, Charles Ehresmann and Paulette Libermann initiated the systematic study of symplectic manifolds as even-dimensional spaces equipped with compatible structures, with Libermann proving the Darboux theorem in the early 1950s to establish local canonical forms.^[6] Concurrently, in the 1950s, André Lichnerowicz and Jean-Louis Koszul contributed to the integration of symplectic structures with Poisson geometry and algebraic frameworks, formalizing them within broader differential and algebraic geometry contexts.^[6] The field experienced a major surge in the 1960s and 1970s, transitioning from classical mechanics to a vibrant area of pure mathematics known as symplectic topology. Vladimir I. Arnold's 1965 paper marked a pivotal moment by demonstrating topological obstructions to certain symplectic embeddings, inspiring applications to dynamical systems and caustics.^[6] This era also saw key works by Jean-Marie Souriau, Bertram Kostant, Victor Guillemin, Shlomo Sternberg, Alan Weinstein, and Jerrold Marsden, who developed concepts like moment maps and symplectic reduction, bridging geometry, physics, and representation theory.^[6] The modern phase began with Mikhail Gromov's 1985 introduction of pseudoholomorphic curves, revolutionizing the study of global symplectic invariants and rigidity phenomena.^[5]

Physical and Geometric Motivation

Symplectic manifolds originate from efforts to formalize classical mechanics, with roots in the early 19th-century works of Siméon-Denis Poisson and William Rowan Hamilton on Hamiltonian systems.^[7] In this context, a symplectic manifold serves as the phase space for a mechanical system, a 2n-dimensional space coordinatized by positions q_i and momenta p_i, where the dynamics of the system are governed by Hamilton's equations. This structure naturally encodes the evolution of physical states, such as the motion of particles under conservative forces. A key feature is Liouville's theorem, which states that the phase flow preserves the volume in this space, ensuring that the density of trajectories remains constant over time and reflecting the incompressibility of phase space under Hamiltonian evolution.^[8]^[9] Geometrically, the symplectic form on such a manifold—a closed, non-degenerate, antisymmetric 2-form—provides an invariant way to measure oriented areas and volumes, distinct from the length and angle measurements afforded by Riemannian metrics. While a Riemannian metric defines distances and curvatures through a symmetric bilinear form, the symplectic form emphasizes the pairing between positions and momenta, enabling a canonical orientation and volume element derived from powers of the form itself. This invariance under symplectomorphisms preserves essential geometric features, such as the symplectic volume \omega^n / n!, which contrasts with the more rigid local invariants in Riemannian geometry.^[10]^[8] The symplectic structure directly ties to conservation laws in mechanics, as Hamiltonian flows—generated by a Hamiltonian function H—preserve the symplectic form, thereby maintaining the symplectic volume along trajectories. This preservation under the flow implies that quantities like energy are conserved when the Hamiltonian is time-independent, and it underpins the long-term stability of dynamical systems by preventing volume collapse or expansion. Such properties ensure that perturbations in mechanical systems do not lead to artificial dissipation, a crucial aspect for accurate modeling of reversible processes.^[9]^[8] Beyond mechanics, symplectic manifolds find applications in optics, where the cotangent bundle models light ray propagation and Hamiltonian ray tracing preserves phase space volumes for lens systems and beam dynamics. In celestial mechanics, they describe planetary orbits and perturbation theories, with symplectomorphisms capturing invariant tori in multi-body problems like the restricted three-body problem. Additionally, the framework bridges to quantum mechanics, where classical phase spaces inform semiclassical approximations and quantization procedures, facilitating transitions between classical and quantum descriptions of systems like the harmonic oscillator.^[9]^[10]

Definition and Basic Properties

Symplectic Form and Axioms

A symplectic manifold is defined as a pair (M, \omega), where M is a smooth manifold of even dimension $2n and \omega is a smooth 2-form on M.^[2] This 2-form \omega serves as a section of the bundle of alternating 2-forms \Lambda^2 T^*M, ensuring compatibility with the smooth structure of M.^[2] The defining axioms for \omega to be a symplectic form are closedness and non-degeneracy. Closedness requires that \omega is a closed differential form, meaning its exterior derivative vanishes: d\omega = 0.^[2] Non-degeneracy stipulates that, at every point p \in M, the interior product map v \mapsto \iota_v \omega_p induces an isomorphism from the tangent space T_p M to its dual T_p^* M, or equivalently, for every nonzero vector v \in T_p M, there exists a vector w \in T_p M such that \omega_p(v, w) \neq 0.^[2] This condition implies that the nth power of the 2-form, \omega^n, is nonzero at each point p, providing a volume form on M and confirming the even dimensionality.^[2] These axioms arise naturally in the context of phase spaces in classical mechanics, where \omega encodes the canonical structure for position and momentum variables.^[2]

Non-degeneracy and Closedness

The closedness condition on a symplectic form \omega, meaning d\omega = 0, ensures that \omega represents a well-defined element [\omega] in the second de Rham cohomology group H^2(M; \mathbb{R}) of the manifold M.^[2] This cohomology class is a global invariant of the symplectic structure, capturing topological obstructions to the existence of \omega as an exact form on M.^[2] Locally, the closedness implies exactness by the Poincaré lemma: on any contractible open set U \subset M, there exists a 1-form \alpha such that \omega|_U = d\alpha.^[2] Non-degeneracy of \omega at each point p \in M means that the associated bilinear form \omega_p: T_p M \times T_p M \to \mathbb{R} is skew-symmetric and invertible, i.e., if \omega_p(v, w) = 0 for all w \in T_p M, then v = 0.^[11] This invertibility follows from the map v \mapsto \iota_v \omega_p being an isomorphism T_p M \to T_p^* M, which requires \dim M = 2n to be even, as skew-symmetric non-degenerate forms exist only on even-dimensional spaces.^[11] A key consequence of non-degeneracy is the existence of a compatible almost complex structure J on M, which satisfies J^2 = -\mathrm{Id} and pairs with \omega to define a Riemannian metric g via \omega(X, Y) = g(JX, Y) for vector fields X, Y, with g positive definite.^[2] Such a J is \omega-compatible, meaning \omega(JX, JY) = \omega(X, Y) and \omega(X, JX) > 0 for X \neq 0.^[2] Together, closedness and non-degeneracy imply that the n-fold wedge product \omega^n is a nowhere-vanishing volume form on the $2n-dimensional manifold M, specifically \frac{\omega^n}{n!} provides a non-zero top-degree form at every point.^[11] On a compact symplectic manifold, this yields a finite total volume \int_M \frac{\omega^n}{n!} < \infty, endowing M with a canonical orientation and Liouville measure preserved by symplectomorphisms.^[2]

Fundamental Theorems

Darboux Theorem

The Darboux theorem asserts that if (M, \omega) is a symplectic manifold of dimension $2n, then for every point p \in M, there exists a neighborhood Uofpand local coordinates(q^1, \dots, q^n, p_1, \dots, p_n)onU$ such that

\omega = \sum_{i=1}^n \, dq^i \wedge dp_i

on U.^[12] These coordinates are known as Darboux coordinates, and the corresponding symplectomorphism to a standard open set in \mathbb{R}^{2n} with the canonical symplectic form highlights the absence of local invariants in symplectic geometry beyond the manifold's dimension.^[12] This result was established by Gaston Darboux in his 1882 memoir addressing the Pfaff problem in differential geometry.^[13] Darboux's original proof, set within the context of integrable Pfaffian systems, demonstrated the local normal form for non-degenerate closed 2-forms, laying foundational groundwork for modern symplectic geometry.^[13] A sketch of the proof begins with the closedness of \omega (d\omega = 0), which, by the Poincaré lemma on contractible neighborhoods, guarantees the existence of a local 1-form \alpha satisfying \omega = d\alpha.^[12] The non-degeneracy of \omega then enables the construction of Darboux coordinates through flows generated by Hamiltonian vector fields: starting from coordinates where \omega matches the standard form at p, time-dependent Hamiltonian flows adjust the 1-form \alpha step-by-step to the canonical \sum p_i \, dq^i, yielding the desired local symplectomorphism.^[12] The theorem implies that all symplectic manifolds of the same dimension are locally symplectomorphic to one another, underscoring a profound uniformity in their local structure that contrasts sharply with Riemannian geometry, where local invariants like the Riemann curvature tensor distinguish manifolds.^[14] This local indistinguishability facilitates global constructions and analyses in symplectic topology by reducing problems to standard models.^[14]

Moser Isotopy Theorem

The Moser isotopy theorem asserts that on a compact smooth manifold M of dimension $2n, if \omega_t for t \in [0,1] is a smooth family of symplectic forms such that the de Rham cohomology class [\omega_t] \in H^2(M; \mathbb{R}) is independent of t, then there exists a smooth isotopy of diffeomorphisms \phi_t: M \to M with \phi_0 = \mathrm{id} satisfying \phi_t^* \omega_t = \omega_0 for all t \in [0,1].^[2]^[15] This implies that \phi_1: (M, \omega_1) \to (M, \omega_0) is a symplectomorphism, classifying such symplectic structures up to global diffeomorphism within their cohomology class.^[2] The proof proceeds via Moser's trick, constructing a time-dependent vector field X_t on M whose time-(t,0)-flow generates the desired isotopy \phi_t. Differentiating \phi_t^* \omega_t = \omega_0 with respect to t yields the homotopy equation

\frac{d}{dt} (\phi_t^* \omega_t) = \phi_t^* \left( \mathcal{L}_{X_t} \omega_t + \frac{\partial}{\partial t} \omega_t \right) = 0,

where \mathcal{L}_{X_t} denotes the Lie derivative along X_t = \frac{d\phi_t}{dt} \circ \phi_t^{-1}.^[2]^[15] Since each \omega_t is closed, \frac{\partial}{\partial t} \omega_t is also closed, and the constant cohomology class ensures [\frac{\partial}{\partial t} \omega_t] = 0, so \frac{\partial}{\partial t} \omega_t = d\mu_t for some smooth 1-form \mu_t obtained via a homotopy operator (e.g., using a partition of unity on the compact manifold to integrate the closed form).^[2] The non-degeneracy of \omega_t then guarantees a unique solution to the contraction equation i_{X_t} \omega_t = -\mu_t, as \omega_t provides an isomorphism between vector fields and 1-forms.^[15] The compactness of M ensures the time-dependent flow of X_t exists and is smooth, yielding the isotopy.^[2] A key application is the stability of symplectic structures: if \omega_1 is sufficiently C^\infty-close to \omega_0, the convex path \omega_t = (1-t)\omega_0 + t \omega_1 consists of symplectic forms (as small perturbations preserve non-degeneracy), so the theorem provides a diffeotopy deforming \omega_1 to \omega_0, demonstrating robustness under perturbations.^[15] This stability underpins the classification of compact symplectic manifolds up to symplectomorphism by their cohomology class and facilitates techniques like symplectic reduction.^[2] The Darboux theorem emerges as a local analogue of this global isotopy result.^[15]

Examples

Symplectic Vector Spaces

A symplectic vector space is a pair (V, \omega), where V is a finite-dimensional vector space over \mathbb{R} of even dimension $2n, and \omega: V \times V \to \mathbb{R} is a skew-symmetric bilinear form that is non-degenerate, meaning that if \omega(v, \cdot) = 0 for all v \in V, then v = 0.^[15] This non-degeneracy implies that the map v \mapsto \omega(v, \cdot) is an isomorphism from V to its dual V^*.^[15] Such spaces form the algebraic foundation for the local structure of symplectic manifolds, modeling their tangent spaces at each point.^[16] In coordinates, every symplectic vector space admits a symplectic basis \{e_1, \dots, e_n, f_1, \dots, f_n\} such that \omega(e_i, f_j) = \delta_{ij} and \omega(e_i, e_j) = \omega(f_i, f_j) = 0.^[15] Relative to this basis, the standard symplectic form is expressed as

\omega = \sum_{i=1}^n e_i^* \wedge f_i^*,

where \{e_i^*, f_i^*\} is the dual basis.^[15] Identifying V with \mathbb{R}^{2n} via this basis, \omega takes the concrete form \sum_{i=1}^n dq_i \wedge dp_i, where q_i and p_i are the coordinate functions corresponding to the e_i and f_i.^[15] This Darboux-like representation highlights the form's role in phase space mechanics.^[16] The linear automorphisms of V that preserve \omega form the symplectic group \mathrm{Sp}(2n, \mathbb{R}), defined as the subgroup of \mathrm{GL}(2n, \mathbb{R}) consisting of matrices A satisfying A^T J A = J, where J = \begin{pmatrix} 0 & I_n \\ -I_n & 0 \end{pmatrix} is the standard symplectic matrix.^[15] This group acts transitively on the set of symplectic bases, ensuring that all symplectic vector spaces of the same dimension are isomorphic.^[15] A key property of symplectic vector spaces is the existence of Lagrangian subspaces, which are maximal isotropic subspaces of dimension n.^[15] A subspace L \subseteq V is isotropic if \omega(u, v) = 0 for all u, v \in L, and it is maximal if no larger subspace satisfies this condition; equivalently, L^\perp = L, where L^\perp = \{ w \in V \mid \omega(w, l) = 0 \ \forall l \in L \}.^[15] For example, the subspace spanned by \{e_1, \dots, e_n\} is Lagrangian with respect to the standard form.^[15] Every isotropic subspace has dimension at most n, and any two Lagrangian subspaces are related by a symplectic transformation.^[15]

Cotangent Bundles

The cotangent bundle of any smooth manifold Q of dimension n provides a canonical example of a symplectic manifold of dimension $2n. The total space T^*Q inherits a natural symplectic structure from the bundle construction itself, independent of any additional data on Q. This structure arises from the tautological 1-form \theta on T^*Q, defined intrinsically as follows: for a point p = (q, \xi) \in T^*Q with q \in Q and \xi \in T_q^*Q, and for any tangent vector v \in T_p(T^*Q), \theta_p(v) = \xi(d\pi_p(v)), where \pi: T^*Q \to Q is the bundle projection map. The associated symplectic form is then \omega = -d\theta, which is closed (d\omega = 0) and non-degenerate on T^*Q, endowing it with the required symplectic axioms.^[2] In local coordinates on T^*Q, choose coordinates (q^i)_{i=1}^n on an open set in Q; the induced coordinates on the bundle are (q^i, p_i)_{i=1}^n, where the p_i parameterize the cotangent fibers. The tautological 1-form takes the expression

\theta = \sum_{i=1}^n p_i \, dq^i,

and the symplectic form is

\omega = \sum_{i=1}^n dq^i \wedge dp_i.

These coordinate expressions confirm the closedness and non-degeneracy locally, as the matrix of \omega with respect to the basis \{dq^i, dp_i\} is the standard block-diagonal form with J = \begin{pmatrix} 0 & I_n \\ -I_n & 0 \end{pmatrix}, which has full rank. The form \omega is globally well-defined and independent of the choice of coordinates, ensuring the symplectic structure is canonical.^[2] Key submanifolds of (T^*Q, \omega) exhibit Lagrangian properties. The zero section, embedded as q \mapsto (q, 0) \in T^*Q, is a Lagrangian submanifold diffeomorphic to Q, since the pullback of \omega to it vanishes identically (as \theta restricts to zero there, so d\theta does as well). Each fiber \pi^{-1}(q) \cong T_q^*Q is a Lagrangian affine subspace, as \omega also restricts to zero on tangent vectors within the fiber (which lie in the dp_i directions, where dq^i = 0). These examples illustrate how the symplectic geometry of T^*Q naturally accommodates maximal-dimensional isotropic submanifolds.^[2] In classical mechanics, T^*Q models the phase space of a system with configuration space Q, where points (q, p) represent generalized positions and conjugate momenta. The canonical symplectic form \omega dictates the dynamics through Hamilton's equations for a Hamiltonian function H: T^*Q \to \mathbb{R}, such as the kinetic-plus-potential energy H(q, p) = \frac{1}{2} |p|^2 + V(q), yielding \dot{q}^i = \frac{\partial H}{\partial p_i} and \dot{p}_i = -\frac{\partial H}{\partial q^i}. This framework unifies the geometric and variational aspects of mechanical systems.

Kähler Manifolds

A Kähler manifold is a complex manifold (M, J) equipped with a Hermitian metric g such that the associated Kähler form \omega(X, Y) = g(JX, Y) is closed, d\omega = 0, thereby endowing M with a symplectic structure inherited from its complex geometry.^[17] Locally, in holomorphic coordinates \{z^j\}, the metric g takes the form g = \sum g_{j\bar{k}} dz^j \otimes d\bar{z}^k with g_{j\bar{k}} = \overline{g_{k\bar{j}}}, and the Kähler form is expressed as

\omega = \frac{i}{2} \sum_{j,k} g_{j\bar{k}} \, dz^j \wedge d\bar{z}^k,

where the coefficients g_{j\bar{k}} form a positive definite Hermitian matrix, ensuring the form is of type (1,1).^[18] The closedness of \omega follows from the relation \omega = \frac{i}{2} \partial \bar{\partial} \phi for some local Kähler potential \phi, as \partial \bar{\partial} + \bar{\partial} \partial = 0.^[17] The triple (g, J, \omega) is compatible in the sense that g is a Riemannian metric invariant under J, J is an integrable almost complex structure defining the complex structure on M, and \omega satisfies \omega(JX, JY) = \omega(X, Y) for all vector fields X, Y, with g(X, Y) = \omega(X, JY).^[19] This compatibility implies that \omega is non-degenerate, as the positivity of g ensures \omega^n \neq 0 everywhere on the $2n-dimensional manifold.[17] The symplectic nature arises directly from the complex structure, distinguishing Kähler manifolds among symplectic ones by requiring integrability of J$. Prominent examples include complex tori, such as \mathbb{T}^{2n} = \mathbb{C}^n / \Lambda with the flat Hermitian metric induced from \mathbb{C}^n, yielding a constant symplectic form \omega = \sum dx_i \wedge dy_i.^[18] Another key example is complex projective space \mathbb{CP}^n, equipped with the Fubini-Study metric, whose Kähler form in affine coordinates U_0 = \{[1:z_1:\dots:z_n]\} is

\omega_\mathrm{FS} = \frac{i}{2} \partial \bar{\partial} \log(1 + |z|^2),

which is invariant under the unitary group U(n+1) action and restricts to the standard form on linear subspaces.^[17] A fundamental property is that the cohomology class [\omega] \in H^2(M, \mathbb{R}) lies in the (1,1)-part of the Dolbeault cohomology H^{1,1}(M) and is positive, known as the Kähler class, which plays a central role in the topology and deformation theory of the manifold.^[18] This class remains fixed under small deformations of the complex structure while preserving the Kähler condition.^[17]

Submanifolds

Lagrangian Submanifolds

In a symplectic manifold (M^{2n}, \omega) of dimension $2n, a submanifold L \subset M is called Lagrangian if it has dimension n and the symplectic form pulls back to zero on L, that is, \iota^*\omega = 0, where \iota: L \to M denotes the inclusion map.^[20] This condition implies that L is isotropic, meaning \omega vanishes on tangent vectors to L, and the dimension condition ensures it is maximal isotropic, or coisotropic as well.^[20] Equivalently, L is Lagrangian if, at every point p \in L, the tangent space T_p L is a Lagrangian subspace of the symplectic vector space (T_p M, \omega_p).^[20] A subspace V \subset T_p M is Lagrangian if it is isotropic (\omega_p(V, V) = 0) and has dimension n, which forces V to be equal to its symplectic orthogonal complement V^\omega = \{ w \in T_p M \mid \omega_p(v, w) = 0 \ \forall v \in V \}.^[20] Standard examples of Lagrangian submanifolds arise in cotangent bundles (T^*Q, \omega_{\rm can}), where \omega_{\rm can} = -d\theta is the canonical symplectic form and \theta is the tautological 1-form. The zero section Z = \{ (q, 0) \in T^*Q \} and each cotangent fiber T^*_q Q are Lagrangian, as \omega_{\rm can} vanishes on their tangent spaces.^[20] More generally, the graph of a closed 1-form \sigma: Q \to T^*Q, given by \Gamma_\sigma = \{ (q, \sigma_q) \in T^*Q \}, is Lagrangian whenever d\sigma = 0, since the pullback of \omega_{\rm can} to \Gamma_\sigma equals d\sigma.^[20] In particular, graphs of exact 1-forms df for smooth functions f: Q \to \mathbb{R} provide exact Lagrangian submanifolds.^[20] A fundamental local structure theorem for Lagrangian submanifolds is Weinstein's Lagrangian neighborhood theorem, which asserts that if L \subset (M, \omega) is a compact Lagrangian submanifold, then there exist neighborhoods U \subset M of L and V \subset T^*L of the zero section in the cotangent bundle of L (equipped with its canonical symplectic form), together with a symplectomorphism \phi: (V, \omega_{\rm can}) \to (U, \omega) that extends the inclusion of the zero section into T^*L.^[2] This result, originally proved in 1971, shows that every Lagrangian submanifold is locally symplectomorphic to the zero section of its own cotangent bundle, providing a normal form for neighborhoods of Lagrangians.^[2]

Special Lagrangian Submanifolds

In a Calabi-Yau manifold (M, \omega, g, J, \Omega), where M is a compact Kähler manifold of complex dimension n with trivial canonical bundle and \Omega a nowhere-vanishing holomorphic volume form normalized so that |\Omega| = 1 with respect to the metric g, a special Lagrangian submanifold is an n-dimensional oriented submanifold L \subset M that is Lagrangian (meaning \omega|_L = 0) and calibrated by the real part of \Omega, satisfying \operatorname{Re}(\Omega)|_L = \operatorname{vol}_g(L), where \operatorname{vol}_g denotes the Riemannian volume form on L induced by g.^[21] This calibration condition implies that L is a \phi-submanifold for \phi = \operatorname{Re}(\Omega), a closed n-form of comass 1 on M.^[21] Special Lagrangian submanifolds minimize area among all Lagrangian submanifolds homologous to them, as calibrated submanifolds are absolutely volume-minimizing in their homology class.^[21] They are minimal submanifolds, meaning their mean curvature vanishes, and they remain stable under small deformations within the space of Lagrangians, providing critical points for the area functional.^[21] The deformation theory of special Lagrangians is obstructed only by the Maslov class, and their moduli spaces are expected to be smooth of the expected dimension when unobstructed. The Thomas-Yau conjecture posits that in a Calabi-Yau manifold, a Lagrangian submanifold is \sigma-stable (for some phase \sigma) if and only if it admits a special Lagrangian representative in its isotopy class, and that the Lagrangian mean curvature flow starting from such a submanifold converges to a special Lagrangian in finite time.^[22] This stability condition is analogous to \mu-stability in algebraic geometry and governs the existence and uniqueness of unobstructed special Lagrangians, with implications for mirror symmetry and enumerative invariants.^[22] Representative examples include the real locus \mathbb{R}^n \subset \mathbb{C}^n, defined by \operatorname{Im}(z_i) = 0 for i=1,\dots,n, which is a flat special Lagrangian submanifold calibrated by \operatorname{Re}(dz_1 \wedge \cdots \wedge dz_n).^[21] Another example is the Clifford torus in the Calabi-Yau torus (\mathbb{C}^n / \Lambda, \operatorname{Re}(\Omega)), given by the product of circles \prod_{i=1}^n \{z_i : |z_i| = 1/\sqrt{n}\} modulo the lattice \Lambda = \mathbb{Z}^{2n}, which is special Lagrangian for the standard holomorphic form.^[21]

Other Coisotropic and Isotropic Submanifolds

In symplectic geometry, an isotropic submanifold L of a symplectic manifold (M^{2n}, \omega) is a submanifold such that the symplectic form \omega vanishes when restricted to the tangent space at every point, i.e., \omega|_ {T_p L} \equiv 0 for all p \in L, or equivalently, T_p L \subseteq (T_p L)^\omega where (T_p L)^\omega = \{ v \in T_p M \mid \omega(v, w) = 0 \ \forall w \in T_p L \}.^[2] This condition implies that the dimension of L satisfies \dim L \leq n.^[2] Isotropic submanifolds arise naturally in contexts such as the orbits of Hamiltonian torus actions on symplectic manifolds.^[2] A coisotropic submanifold C satisfies the dual condition (T_p C)^\omega \subseteq T_p C for all p \in C, which is equivalent to the conormal bundle N^* C being contained in T C, or the annihilator of T_p C in T_p^* M lying inside T_p C. Consequently, \dim C \geq n. For coisotropic submanifolds, the distribution (T C)^\omega is integrable, defining a characteristic foliation whose leaves are symplectic submanifolds of C, known as symplectic leaves; the quotient space C / ((T C)^\omega) inherits a symplectic structure induced by \omega.^[2] Examples of isotropic submanifolds include points in M, where the tangent space is zero-dimensional and thus trivially satisfies \omega \equiv 0, and one-dimensional submanifolds (curves) in any symplectic manifold, as their tangent spaces are one-dimensional and \omega pairs vectors to scalars.^[23] In the standard symplectic vector space \mathbb{R}^{2n} with the canonical form, the subspace spanned by the first two standard basis vectors e_1, e_2 is isotropic.^[2] For coisotropic submanifolds, codimension-one subspaces (hypersurfaces) in \mathbb{R}^{2n} qualify, as their symplectic orthogonal is one-dimensional and contained within them due to the nondegeneracy of \omega.^[2] In low dimensions, such as n=1 where M is a surface, any hypersurface is coisotropic by dimension.^[23] Coisotropic and isotropic submanifolds are central to the study of Dirac structures, which provide a unified framework for symplectic and Poisson geometries within generalized complex geometry. A Dirac structure on M is a maximal isotropic subbundle of the Courant algebroid TM \oplus T^*M that is closed under the Courant bracket; in the symplectic case, the graph of \omega defines such a structure.^[24] Submanifolds whose normal bundles are isotropic or coisotropic with respect to the tangent Dirac structure induced by this framework characterize Dirac submanifolds, enabling extensions of symplectic reduction and embeddings to more general settings.^[24] This connection facilitates the analysis of coisotropic embeddings and their role in Dirac manifolds.^[24]

Advanced Structures

Lagrangian Fibrations

A Lagrangian fibration on a symplectic manifold (M, \omega) of dimension $2nis defined as a surjective submersion\pi: M \to Bonto ann-dimensional manifold B, such that the connected components of the regular fibers \pi^{-1}(b)are [Lagrangian](/page/Lagrangian) submanifolds ofM.[26] These fibrations often come equipped with a global section, allowing Mto be viewed as a [fiber bundle](/page/Fiber_bundle) overB$ with Lagrangian fibers.^[25] The fibers are typically compact tori in regular regions, reflecting the structure of integrable systems.^[26] Lagrangian fibrations are closely tied to completely integrable Hamiltonian systems on (M, \omega). In such systems, a momentum map F = (f_1, \dots, f_n): M \to \mathbb{R}^n consisting of n Poisson-commuting Hamiltonians \{f_i, f_j\} = 0 with linearly independent differentials almost everywhere induces a Lagrangian fibration via the projection to \mathbb{R}^n, where the level sets F^{-1}(c) form the Lagrangian fibers.^[26] Near a regular value c \in \mathbb{R}^n with compact connected fiber, the Liouville-Arnold theorem guarantees the existence of action-angle coordinates: local symplectic coordinates (I_1, \dots, I_n, \theta_1, \dots, \theta_n) where the I_k = f_k are the action variables (constants along the flow) and the \theta_k are angle variables parametrizing the n-torus fiber.^[26] Prominent examples include the canonical projection of the cotangent bundle (T^*B, \Omega_{\mathrm{can}}) \to B for any manifold B, where the zero section provides a natural choice of Lagrangian fibers diffeomorphic to B itself, though typically non-compact unless B is.^[26] Another class arises from moment maps in symplectic toric manifolds: for a compact connected $2n-dimensional symplectic manifold admitting an effective Hamiltonian T^n-action, the moment map \mu: M \to \mathbb{R}^nyields a Lagrangian fibration withn$-torus fibers over the interior of the image (a Delzant polytope), classifying such manifolds up to equivariant symplectomorphism.^[25] Singularities in Lagrangian fibrations occur at critical values of \pi, disrupting the regular torus structure. A key type is the focus-focus singularity, characterized locally by a fiber consisting of two Lagrangian disks intersecting transversely at a pinch point, as modeled by the map (z,w) \mapsto zw from \mathbb{C}^2 to \mathbb{C}.^[26] These singularities induce non-trivial monodromy in the fibration: as one encircles a focus-focus critical value in the base, the homology cycles of nearby torus fibers undergo a shear transformation, obstructing global action-angle coordinates and reflecting quantum mechanical effects like those in the spherical pendulum.^[26]

Symplectic Reduction

Symplectic reduction is a fundamental technique in symplectic geometry for constructing new symplectic manifolds from those possessing symmetries, particularly through the quotienting of level sets of momentum maps associated to group actions. In the context of a symplectic manifold (M, \omega) equipped with a Hamiltonian action of a Lie group G, the momentum map \mu: M \to \mathfrak{g}^* encodes the infinitesimal symmetries, allowing the reduction process to eliminate redundant degrees of freedom while preserving the symplectic structure. This method, known as Marsden-Weinstein reduction, yields a reduced space that inherits a natural symplectic form, facilitating the study of dynamical systems with conserved quantities.^[27] The Marsden-Weinstein reduction theorem states that if G acts freely and properly on the level set \mu^{-1}(0) \subset M, then the reduced space \mu^{-1}(0)/G is a smooth symplectic manifold with the reduced symplectic form \omega_{\mathrm{red}} defined as follows: for \in \mu^{-1}(0)/G and tangent vectors X_{\mathrm{red}}, Y_{\mathrm{red}} at $$, choose representatives \tilde{X}, \tilde{Y} \in T_p M that are tangent to a G-invariant slice through p, and set

\omega_{\mathrm{red}}()(X_{\mathrm{red}}, Y_{\mathrm{red}}) = \omega_p(\tilde{X}, \tilde{Y}).

This form is well-defined, independent of the choice of slice, and closed, making (\mu^{-1}(0)/G, \omega_{\mathrm{red}}) symplectic. The theorem assumes the action is symplectic and Hamiltonian, with \mu equivariant, ensuring the reduced dynamics correspond to the original via the symplectic quotient.^[27] A classic example arises in the Kepler problem, where the phase space is the cotangent bundle T^*\mathbb{R}^3 \setminus \{0\} with the standard symplectic form, and the Hamiltonian describes a particle in a $1/r potential under SO(3) rotational symmetry. The momentum map \mu corresponds to angular momentum, and reduction at \mu^{-1}(0) (zero angular momentum, or radial motion) yields a singular reduced space topologically equivalent to a cylinder, capturing the radial dynamics of the system.^[28] Further SO(4) hidden symmetry reduction at non-zero levels produces a spherical phase space S^2 \times S^2, capturing the bounded orbits as geodesic flow on this quotient. This reduction reveals the integrability and closed orbit structure of the system.^[28] Another prominent example is the construction of coadjoint orbits as reduced spaces. For a Lie group G acting on its dual Lie algebra \mathfrak{g}^* via the coadjoint action, the momentum map for the lifted action on T^*G identifies coadjoint orbits O_\xi = \mathrm{Ad}^*_G \xi \subset \mathfrak{g}^* with symplectic reductions \mu^{-1}(\xi)/G_\xi, where G_\xi is the co-stabilizer. The induced Kirillov-Kostant-Souriau symplectic form on O_\xi is given by \omega_\xi(\hat{X}, \hat{Y}) = -\xi([X, Y]) for infinitesimal generators \hat{X}, \hat{Y}, making these orbits the basic building blocks for the orbit method in representation theory. More generally, reduction can occur at regular coadjoint values \xi \in \mathfrak{g}^* where the coadjoint action is locally free, yielding the reduced space \mu^{-1}(\xi)/G_\xi as a symplectic manifold. Here, G_\xi is the stabilizer subgroup, and the reduced form is defined analogously on slices transverse to the G_\xi-orbits, preserving the symplectic structure for arbitrary regular levels beyond the zero level. This extension applies to systems with non-trivial conserved momenta, such as in rigid body dynamics or celestial mechanics.^[27]

Hamiltonian Group Actions

A Lie group G acts symplectically on a symplectic manifold (M, \omega) if the action preserves the symplectic form \omega, meaning that the induced diffeomorphisms satisfy g^* \omega = \omega for all g \in G. Such an action is called Hamiltonian if there exists a smooth map \mu: M \to \mathfrak{g}^*, called the momentum map, where \mathfrak{g}^* is the dual of the Lie algebra \mathfrak{g} of G, satisfying the condition

d \langle \mu, \xi \rangle = -\iota_{X_\xi} \omega

for every \xi \in \mathfrak{g}, with X_\xi denoting the infinitesimal generator of the action corresponding to \xi and \iota the interior product.^[29] The momentum map \mu is equivariant with respect to the coadjoint action of G on \mathfrak{g}^*, meaning \mu(g \cdot m) = \mathrm{Ad}^*_g \mu(m) for all g \in G and m \in M, where \mathrm{Ad}^* is the coadjoint representation. This equivariance ensures that the Hamiltonian action integrates the symmetries of the system consistently with the Poisson structure induced by \omega.^[29] A prominent example of a Hamiltonian group action is the action of the torus T^n on a compact toric symplectic manifold, where the momentum map \mu: M \to \mathbb{R}^n has an image that is a convex polytope, as established by the Atiyah-Guillemin-Sternberg convexity theorem. Another example is the linear action of the symplectic group \mathrm{Sp}(2n, \mathbb{R}) on \mathbb{R}^{2n} equipped with the standard symplectic form, where the momentum map is given by \langle \mu(x), \xi \rangle = \frac{1}{2} \omega(x, \xi x) for x \in \mathbb{R}^{2n} and \xi \in \mathfrak{sp}(2n, \mathbb{R}), satisfying the defining condition for each element of the Lie algebra.^[30] The Duistermaat-Heckman theorem provides a key quantitative result for Hamiltonian actions of compact Lie groups on compact symplectic manifolds, stating that the pushforward under the momentum map of the normalized Liouville measure (defined by \omega^n / n!) is a piecewise polynomial function on \mathfrak{g}^* of degree at most \dim M / 2 - \dim G, with the volume of the reduced spaces at regular values varying linearly across chambers separated by walls of codimension one. This theorem highlights the polynomial nature of the measure on the reduced phase spaces and has implications for quantization and cohomology.^[31]

Generalizations and Extensions

Almost Symplectic Manifolds

An almost symplectic manifold is a smooth even-dimensional manifold M^{2n} equipped with a non-degenerate 2-form \omega, meaning that for every point p \in M, the interior product map v \mapsto i_v \omega_p: T_p M \to T_p^* M is a vector bundle isomorphism, but \omega is not required to be closed (d\omega \neq 0). This structure relaxes the closedness axiom of standard symplectic manifolds while preserving non-degeneracy, which ensures that \omega induces a pairing on the tangent spaces with maximal rank. Such manifolds arise naturally in contexts like nonholonomic mechanics, where the failure of closedness reflects constraints that prevent exact energy conservation.^[32] The non-degeneracy of \omega allows the construction of compatible almost complex structures J on M. Specifically, there exists a Riemannian metric g such that g(u, v) = \omega(u, J v), J^2 = -Id, and \omega(J u, J v) = \omega(u, v) for all tangent vectors u, v, forming an almost Hermitian triple (\omega, J, g). The space of such compatible almost complex structures is non-empty and contractible on compact manifolds. Integrability of J—meaning it defines a complex structure—is obstructed by the Nijenhuis tensor N_J, defined by

N_J(X, Y) = [X, Y] + J[JX, Y] + J[X, JY] - [JX, JY]

for vector fields X, Y; N_J = 0 if and only if J is integrable, by the Newlander–Nirenberg theorem. In the almost Hermitian setting induced by \omega, N_J relates to d\omega through the torsion of the canonical connection: the (3,0) + (0,3) components of d\omega contribute to the skew-symmetric part of N_J, providing an obstruction to both complex and symplectic integrability.^[33]^[34] A canonical example of almost symplectic manifolds are nearly Kähler manifolds, which are almost Hermitian manifolds satisfying (\nabla_X J)X = 0 for all vector fields X, where \nabla is the Levi-Civita connection. Equivalently, d\omega = 3\lambda \operatorname{Re}(\Omega) for some constant \lambda \neq 0 and fundamental (3,0)-form \Omega, ensuring d\omega \neq 0 while maintaining non-degeneracy; the Nijenhuis tensor is totally skew-symmetric and non-vanishing in non-Kähler cases, such as the standard nearly Kähler structure on S^6. These structures appear in supersymmetry and calibrated geometry, highlighting how the specific form of d\omega encodes obstructions to both closedness and integrability. Almost symplectic structures can be deformed to genuine symplectic ones via averaging techniques when a suitable Lie group action exists, such as averaging \omega over the orbits to produce a closed invariant form in the same cohomology class, provided the action is Hamiltonian-like and d\omega lies in the image of the Lie derivative operator. This method leverages the geometry of group actions to resolve the closedness obstruction while preserving non-degeneracy locally.

Contact Manifolds

A contact manifold is an odd-dimensional smooth manifold N of dimension $2n+1 equipped with a contact structure, which can be defined via a contact 1-form \alpha, a nowhere-vanishing differential 1-form satisfying the non-degeneracy condition \alpha \wedge (d\alpha)^n \neq 0.^[35] This condition ensures that the kernel of \alpha, denoted \xi = \ker \alpha, is a hyperplane distribution on which d\alpha restricts to a symplectic form, making contact structures odd-dimensional analogues of symplectic structures.^[35] Two contact 1-forms \alpha and \alpha' define the same contact structure if \alpha' = f \alpha for some positive smooth function f: N \to \mathbb{R}^+.^[35] Contact manifolds relate to symplectic manifolds through the process of symplectization, which constructs an even-dimensional symplectic manifold from a contact one. Specifically, the symplectization of the contact manifold (N, \alpha) is the product manifold N \times \mathbb{R} equipped with the symplectic form \omega = d(e^t \alpha), where t is the coordinate on \mathbb{R}; this \omega is conformally symplectic and compatible with the contact structure in the sense that the hyperplanes \xi lift to the kernel of the primitive 1-form e^t \alpha.^[36] This construction provides a bridge between contact and symplectic geometry, allowing techniques from one field to inform the other, such as embedding problems or dynamical properties.^[36] A canonical example of a contact manifold is the standard contact structure on \mathbb{R}^{2n+1} with coordinates (x_1, \dots, x_n, y_1, \dots, y_n, z), where the contact 1-form is \alpha = dz - \sum_{i=1}^n y_i \, dx^i.^[37] This form satisfies the non-degeneracy condition, as d\alpha = \sum_{i=1}^n dx^i \wedge dy^i restricts to the standard symplectic form on the contact planes \ker \alpha.^[37] The standard contact structure on the unit sphere S^{2n+1} \subset \mathbb{R}^{2n+2} is induced by restricting this form after identifying the sphere with the projectivization of the contact planes.^[37] Associated to any contact 1-form \alpha is the Reeb vector field, a unique nowhere-vanishing vector field \xi on N characterized by the conditions \alpha(\xi) = 1 and \iota_\xi d\alpha = 0.^[38] The Reeb field is transverse to the contact distribution \xi = \ker \alpha and generates a flow that preserves the contact structure, playing a central role in the dynamics of contact manifolds, such as in the study of periodic orbits.^[38] For the standard contact form on \mathbb{R}^{2n+1}, the Reeb vector field is simply \partial_z, reflecting the translational invariance in the z-direction.^[37]

Poisson Manifolds

A Poisson manifold is a smooth manifold M equipped with a bivector field \pi \in \Gamma(\wedge^2 TM) satisfying [\pi, \pi]_S = 0, where [ \cdot, \cdot ]_S denotes the Schouten-Nijenhuis bracket. This condition ensures that the associated bilinear map \{f, g\} = \pi(df, dg) on smooth functions C^\infty(M) defines a Poisson bracket, which is skew-symmetric, satisfies the Leibniz rule \{f, gh\} = g\{f, h\} + h\{f, g\}, and obeys the Jacobi identity. The bivector \pi induces a bundle map \pi^\sharp: T^*M \to TM given by \pi^\sharp(\alpha) = \iota_\alpha \pi, and the Poisson bracket generates Hamiltonian vector fields X_f = \pi^\sharp(df). The image of \pi^\sharp defines an integrable distribution on M, and its integral submanifolds, known as symplectic leaves, are the connected components where \pi restricts to a non-degenerate bivector.^[39] On each symplectic leaf L, the rank of \pi is constant and even, and the inverse of the restricted bivector yields a symplectic form \omega_L such that \pi^\sharp_L = \omega_L^{-1}.^[39] Locally, around a point where the rank of \pi is constant, M decomposes as a product of a symplectic manifold and a transverse manifold, with the leaves corresponding to the symplectic factor.^[39] This foliation generalizes the non-degenerate case of symplectic manifolds, allowing for degeneracy while preserving the Poisson bracket structure. Prominent examples include the dual space \mathfrak{g}^* of a finite-dimensional Lie algebra \mathfrak{g}, endowed with the Lie-Poisson bivector defined by \{f, g\}(\mu) = \langle \mu, [df(\mu), dg(\mu)] \rangle for \mu \in \mathfrak{g}^* and f, g \in C^\infty(\mathfrak{g}^*).^[39] Here, the symplectic leaves are the coadjoint orbits, which carry the Kirillov-Kostant-Souriau symplectic structure.^[39] Another example is the zero Poisson structure \pi = 0 on any smooth manifold, such as a Riemannian manifold, where the bracket vanishes identically \{f, g\} = 0, resulting in zero-dimensional symplectic leaves (points) and trivial dynamics. Casimir functions on a Poisson manifold M are smooth functions C \in C^\infty(M) that Poisson-commute with every function, i.e., \{C, f\} = 0 for all f \in C^\infty(M), or equivalently, dC lies in the kernel of \pi^\sharp.^[40] Such functions are constant along the symplectic leaves, generating the center of the Poisson algebra and serving as invariants that foliate M into level sets.^[40] In the Lie-Poisson case on \mathfrak{g}^*, the Casimirs are the invariant polynomials on \mathfrak{g}, constant on coadjoint orbits.^[40]

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