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On maximal tori in the contactomorphism groups of regular contact manifolds  [PDF]
Eugene Lerman
Mathematics , 2002,
Abstract: By a theorem of Banyaga the group of diffeomorphisms of a manifold $P$ preserving a regular contact form $\alpha$ is a central $S^1$ extension of the commutator of the group of symplectomorphisms of the base $B = P/S^1$. We show that if $T$ is a Hamiltonian maximal torus in the group of symplectomorphism of $B$, then its preimage under the extension map is a maximal torus not only in the group $\Diff(P, \alpha)$ of diffeomorphisms of $P$ preserving $\alpha$ but also in the much bigger group of contactomorphisms $\Diff (P, \xi)$, the group of diffeomorphism of $P$ preserving the contact distribution $\xi = \ker \alpha$. We use this (and the work of Hausmann, and Tolman on polygon spaces) to give examples of contact manifolds $(P, \xi = \ker \alpha)$ with maximal tori of different dimensions in their group of contactomorphisms.
Maximal compact tori in the Hamiltonian groups of 4-dimensional symplectic manifolds  [PDF]
Martin Pinsonnault
Mathematics , 2006,
Abstract: We prove that the group of Hamiltonian automorphisms of a symplectic 4-manifold contains only finitely many conjugacy classes of maximal compact tori with respect to the action of the full symplectomorphism group. We also extend to rational and ruled manifolds a result of Kedra which asserts that, if $M$ is a simply connected symplectic 4-manifold with $b_{2}\geq 3$, and if $\widetilde{M}_{\delta}$ denotes a blow-up of $M$ of small enough capacity $\delta$, then the rational cohomology algebra of the Hamiltonian group of $\widetilde{M}_{\delta})$ is not finitely generated. Both results are based on the fact that in a symplectic 4-manifold endowed with any tamed almost structure $J$, exceptional classes of minimal symplectic area are $J$-indecomposable. Some applications and examples are given.
Big polygon spaces  [PDF]
Matthias Franz
Mathematics , 2014,
Abstract: We study a new class of compact orientable manifolds, called big polygon spaces. They are intersections of real quadrics and related to polygon spaces, which appear as their fixed point set under a canonical torus action. What makes big polygon spaces interesting is that they exhibit remarkable new features in equivariant cohomology: The Chang-Skjelbred sequence can be exact for them and the equivariant Poincare pairing perfect although their equivariant cohomology is never free as a module over the cohomology ring of BT. More generally, big polygon spaces show that a certain bound on the syzygy order of the equivariant cohomology of compact orientable T-manifolds obtained by Allday, Puppe and the author is sharp.
Topological complexity of planar polygon spaces with small genetic code  [PDF]
Donald M. Davis
Mathematics , 2015,
Abstract: We determine, within 1, the topological complexity of many planar polygon spaces mod isometry. In all cases except those homeomorphic to real projective space or n-tori, the upper and lower bounds given by dimension and cohomology considerations differ by 1. The spaces which we consider are those whose genetic codes, in the sense of Hausmann and Rodriguez, have a single gene, and its size is less than 5.
Contact Homology of Hamiltonian Mapping Tori  [PDF]
Oliver Fabert
Mathematics , 2006,
Abstract: This paper is concerned with the rational symplectic field theory in the Floer case. For this observe that in the general geometric setup for symplectic field theory the contact manifolds can be replaced by mapping tori of symplectic manifolds with symplectomorphisms. While the cylindrical contact homology is given by the Floer homologies of powers of the symplectomorphism, the other algebraic invariants of symplectic field theory provide natural generalizations of symplectic Floer homology. For symplectically aspherical manifolds and Hamiltonian symplectomorphisms we study the moduli spaces of rational curves and prove a transversality result, which does not need the polyfold theory by Hofer, Wysocki and Zehnder. Besides that our result shows that one does not get nontrivial operations on Floer homology from symplectic field theory, we use it to compute the full contact homology of the corresponding Hamiltonian mapping torus.
Intersection Numbers of Polygon Spaces  [PDF]
José Agapito,Leonor Godinho
Mathematics , 2007,
Abstract: We study the intersection ring of the space $\M(\alpha_1,...,\alpha_m)$ of polygons in $\R^3$. We find homology cycles dual to generators of this ring and prove a recursion relation in $m$ (the number of steps) for their intersection numbers. This result is analog of the recursion relation appearing in the work of Witten and Kontsevich on moduli spaces of punctured curves and on the work of Weitsman on moduli spaces of flat connections on two-manifolds of genus $g$ with $m$ marked points. Based on this recursion formula we obtain an explicit expression for the computation of the intersection numbers of polygon spaces and use it in several examples. Among others, we study the special case of equilateral polygon spaces (where all the $\alpha_i$ are the same) and compare our results with the expressions for these particular spaces that have been determined by Kamiyama and Tezuka. Finally, we relate our explicit formula for the intersection numbers with the generating function for intersection pairings of the moduli space of flat connections of Yoshida, as well as with equivalent expressions for polygon spaces obtained by Takakura and Konno through different techniques.
Polygon spaces and Grassmannians  [PDF]
Jean-Claude Hausmann,Allen Knutson
Mathematics , 1996,
Abstract: We study the moduli spaces of polygons in R^2 and R^3, identifying them with subquotients of 2-Grassmannians using a symplectic version of the Gel'fand-MacPherson correspondence. We show that the bending flows defined by Kapovich-Millson arise as a reduction of the Gel'fand-Cetlin system on the Grassmannian, and with these determine the pentagon and hexagon spaces up to equivariant symplectomorphism. Other than invocation of Delzant's theorem, our proofs are purely polygon-theoretic in nature.
The cohomology ring of polygon spaces  [PDF]
Jean-Claude Hausmann,Allen Knutson
Mathematics , 1997,
Abstract: We compute the integer cohomology rings of the ``polygon spaces'' introduced in [Hausmann,Klyachko,Kapovich-Millson]. This is done by embedding them in certain toric varieties; the restriction map on cohomology is surjective and we calculate its kernel using ideas from the theory of Gr\"obner bases. Since we do not invert the prime 2, we can tensor with Z/2; halving all degrees we show this produces the Z/2 cohomology rings of planar polygon spaces. In the equilateral case, where there is an action of the symmetric group permuting the edges, we show that the induced action on the integer cohomology is _not_ the standard one, despite it being so on the rational cohomology [Kl]. Finally, our formulae for the Poincar\'e polynomials are more computationally effective than those known [Kl].
Geometric descriptions of polygon and chain spaces  [PDF]
Jean-Claude Hausmann
Mathematics , 2007,
Abstract: We give a few simple methods to geometically describe some polygon and chain-spaces in R^d. They are strong enough to give tables of m-gons and m-chains when m <= 6.
Maximal Tori in Contactomorphism Groups  [PDF]
Charles P. Boyer
Mathematics , 2010,
Abstract: I describe a general scheme which associates conjugacy classes of tori in the contactomorphism group to transverse almost complex structures on a compact contact manifold. Moreover, to tori of Reeb type whose Lie algebra contains a Reeb vector field one can associate a Sasaki cone. Thus, for contact structures of K-contact type one obtains a configuration of Sasaki cones called a bouquet such that each Sasaki cone is associated with a conjugacy class of tori of Reeb type.
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