Abstract. We give an algorithm for computing the contact homology of some Brieskorn manifolds. Brieskorn manifolds can be regarded as circle-bundles over orbifolds and our algorithm expresses cylindrical contact homology of the Brieskorn manifold in terms of the homology of the underlying orbifold. As an application, we construct infinitely many contact structures on the class of simply connected contact manifolds that admit nice contact forms (i.e. no Reeb orbits of degree -1, 0 or 1) and have index positivity with trivial first Chern class.
IntroductionFor a long time it has been known that contact manifolds may carry many nonisomorphic contact structures. A first way to distinguish these structures from each other is by considering their Chern class or their formal homotopy class. In dimension 3 we can, in addition, sometimes distinguish contact structures by showing tightness or overtwistedness as was shown first by Bennequin. At present, the latter two notions do not have generalizations to higher dimensions.Eliashberg was the first to show that the classical invariants (i.e. the Chern class and formal homotopy class) are not always enough for a full classification of contact structures on a given manifold in dimension larger than 3. He showed that spheres in dimensions 4n + 1 admit a non-standard, yet homotopically trivial contact structure in [4]. Giroux gave an interesting example in dimension 3 in [7]. He exhibited infinitely many tight contact structures on T 3 in the same homotopy class of plane fields. Since then more general techniques to distinguish contact structures were introduced. Eliashberg and Hofer's contact homology is such a technique (for a survey, see [5], and for a more recent description, [6]). It works, roughly speaking, as follows. The Reeb orbits on a given contact manifold can be given a grading. The chain complexes are freely generated by the closed Reeb orbits. Then a differential can be defined by counting certain holomorphic curves asymptotic to Reeb orbits in the symplectization of the contact manifold. It can be shown that the homology of this differential is independent of the contact form under suitable conditions and it is hence an invariant of the contact structure. Since its introduction, several examples of non-isomorphic contact structures in the same homotopy class have been found.One of these examples is a family of Brieskorn spheres in dimensions 4n + 1. These were found by Ustilovsky in [14]. In his thesis he used a few other tools that can easily be applied to construct more contact structures on the same manifold. This is done by connect summing. It is well known that the connected sum of two contact manifolds carries again a contact structure, see [11] and [16]. In his thesis 1991 Mathematics Subject Classification. Primary 53D10 .