Finite rigid sets and homologically non-trivial spheres in the curve complex of a surface

Aramayona and Leininger have provided a "finite rigid subset" $\mathfrak{X}(\Sigma)$ of the curve complex $\mathscr{C}(\Sigma)$ of a surface $\Sigma = \Sigma^n_g$, characterized by the fact that any simplicial injection $\mathfrak{X}(\Sigma) \to \mathscr{C}(\Sigma)$ is induced by a unique element of the mapping class group $\mathrm{Mod}(\Sigma)$. In this paper we prove that, in the case of the sphere with $n\geq 5$ marked points, the reduced homology class of the finite rigid set of Aramayona and Leininger is a $\mathrm{Mod}(\Sigma)$-module generator for the reduced homology of the curve complex $\mathscr{C}(\Sigma)$, answering in the affirmative a question posed by Aramayona and Leininger. For the surface $\Sigma = \Sigma_g^n$ with $g\geq 3$ and $n\in \{0,1\}$ we find that the finite rigid set $\mathfrak{X}(\Sigma)$ of Aramayona and Leininger contains a proper subcomplex $X(\Sigma)$ whose reduced homology class is a $\mathrm{Mod}(\Sigma)$-module generator for the reduced homology of $\mathscr{C}(\Sigma)$ but which is not itself rigid.Comment: 21 pages, 7 figures; Section 4 revised along with minor corrections throughou

The Johnson homomorphism and its kernel

We give a new proof of a celebrated theorem of Dennis Johnson that asserts that the kernel of the Johnson homomorphism on the Torelli subgroup of the mapping class group is generated by separating twists. In fact, we prove a more general result that also applies to "subsurface Torelli groups". Using this, we extend Johnson's calculation of the rational abelianization of the Torelli group not only to the subsurface Torelli groups, but also to finite-index subgroups of the Torelli group that contain the kernel of the Johnson homomorphism.Comment: 32 pages, 11 figures; major revision; to appear in J. Reine Angew. Mat

Cofinal elements and fractional Dehn twist coefficients

We show that for a surface $S$ with positive genus and one boundary component, the mapping class of a Dehn twist along a curve parallel to the boundary is cofinal in every left ordering of the mapping class group $\operatorname{Mod}(S)$. We apply this result to show that one of the usual definitions of the fractional Dehn twist coefficient -- via translation numbers of a particular action of $\operatorname{Mod}(S)$ on $\mathbb{R}$ -- is in fact independent of the underlying action when $S$ has genus larger than one. As an algebraic counterpart to this, we provide a formula that recovers the fractional Dehn twist coefficient of a homeomorphism of $S$ from an arbitrary left ordering of $\operatorname{Mod}(S)$.Comment: 13 pages. This version has minor changes to the mathematical content, and substantial changes to the exposition. To appear in IMR

Mapping Class Groups and Moduli Spaces of Curves

This is a survey paper that also contains some new results. It will appear in the proceedings of the AMS summer research institute on Algebraic Geometry at Santa Cruz.Comment: We expanded section 7 and rewrote parts of section 10. We also did some editing and made some minor corrections. latex2e, 46 page

Proper actions, nonlinearity and homotopy theory

In dieser Arbeit wird die Erweiterung der Methoden der äquivarianten stabilen Homotopietheorie zu breiteren Kontexten untersucht. Die klassische Theorie voraussetzt Kompaktheit oder sogar Endlichkeit an der wirkenden Gruppe. Äquivariante Homotopie und Kohomotopie werden durch Spektren und analytische Methoden für eigentliche G-CW Komplexe konstruiert. Die Übereinstimmung mit der klassischen Definition, sowie zu einer von Lück 2005 veröffentlichten Konstruktion mittels Vektorraumbündeln wird bewiesen. Die Segal Vermutung wird in zwei Versionen verallgemeinert (für familien endlicher Untergruppen in diskreten Gruppen, bzw. für halb-einfache Liegruppen deren maximale kompakte Untergruppe keine Fundamentaldarstellung quaternionischen Typs aufweist). Eine bivariante , äquivariante homotopietheorie für C*-Algebren wird auch definiert

A Koszul duality for props

The notion of prop models the operations with multiple inputs and multiple outpus, acting on some algebraic structures like the bialgebras or the Lie bialgebras. In this paper, we generalize the Koszul duality theory of associative algebras and operads to props.Comment: Last version before publication in Transactions of the A.M.S (statement about the properad of involutive Lie bialgebras removed

Singularities, Supersymmetry and Combinatorial Reciprocity

This work illustrates a method to investigate certain smooth, codimension-two, real submanifolds of spheres of arbitrary odd dimension (with complements that fiber over the circle) using a novel supersymmetric quantum invariant. Algebraic (fibered) links, including Brieskorn-Pham homology spheres with exotic differentiable structure, are examples of said manifolds with a relative diffeomorphism-type that is determined by the corresponding (multivariate) Alexander polynomial.Engineering and Applied Science