arXiv subjects

Ergun Yalcin

Publications and source records attributed to Ergun Yalcin.

30 records · Page 2Linked to original sources

arXiv2013

On a canonical construction of tesselated surfaces via finite group theory, Part I

This paper is the first part in a 2 part study of an elementary functorial construction from the category of finite non-abelian groups to a category of singular compact, oriented 2-manifolds. After a desingularization process this construction results in a collection of compact, connected, oriented tesselated smooth surfaces equipped with a closed-cell structure which is face and edge transitive and which has at most 2 orbits of vertices. These tesselated surfaces can also be viewed as abstract 3-polytopes (or as graph embeddings in the corresponding surface) which are either equivar or dual to abstract quasiregular polytopes. This construction generally results in a large collection of tesselated surfaces per group, for example when the construction is applied to Σ_6 it yields 4477 tesselated surfaces of 27 distinct genus and even more varieties of tesselation cell structure. We study the distribution of these surfaces in various groups and some interesting resulting tesselations. In a second paper, we show that extensions of groups result in branched coverings between the component surfaces in their decompositions. We also exploit functoriality to obtain interesting faithful, orientation preserving actions of subquotients of these groups and their automorphism groups on these surfaces and in the corresponding mapping class groups.

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arXiv2012

Free Actions on Products of Spheres at High Dimensions

A classical conjecture in transformation group theory states that if $G=(\bbZ/p)^r$ acts freely on a product of $k$ spheres $S^{n_1} \times ... \times S^{n_k}$, then $r\leq k$. We prove this conjecture in the case where the average of the dimensions ${n_i}$ is large compared to the differences $|n_i-n_j|$ between the dimensions.

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arXiv2012

Relative group cohomology and the orbit category

Let $G$ be a finite group and $\cF$ be a family of subgroups of $G$ closed under conjugation and taking subgroups. We consider the question whether there exists a periodic relative $\cF$-projective resolution for $\bbZ$ when $\cF$ is the family of all subgroups $H \leq G$ with $\rk H \leq \rk G-1$. We answer this question negatively by calculating the relative group cohomology $\cF H^* (G, \bbF_2)$ where $G=\bbZ/2\times \bbZ /2$ and $\cF$ is the family of cyclic subgroups of $G$. To do this calculation we first observe that the relative group cohomology $\cF H^*(G, M)$ can be calculated using the ext-groups over the orbit category of $G$ restricted to the family $\cF$. In second part of the paper, we discuss the construction of a spectral sequence that converges to the cohomology of a group $G$ and whose horizontal line at $E_2$ page is isomorphic to the relative group cohomology of $G$.

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arXiv2012

Bockstein Closed 2-Group Extensions and Cohomology of Quadratic Maps

A central extension of the form $E: 0 \to V \to G \to W \to 0$, where $V$ and $W$ are elementary abelian 2-groups, is called Bockstein closed if the components $q_i \in H^*(W, \FF_2)$ of the extension class of $E$ generate an ideal which is closed under the Bockstein operator. In this paper, we study the cohomology ring of $G$ when $E$ is a Bockstein closed 2-power exact extension. The mod-2 cohomology ring of $G$ has a simple form and it is easy to calculate. The main result of the paper is the calculation of the Bocksteins of the generators of the mod-2 cohomology ring using an Eilenberg-Moore spectral sequence. We also find an interpretation of the second page of the Bockstein spectral sequence in terms of a new cohomology theory that we define for Bockstein closed quadratic maps $Q : W \to V$ associated to the extensions $E$ of the above form.

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arXiv2012

Productive elements in group cohomology

Let $G$ be a finite group and $k$ be a field of characteristic $p>0$. A cohomology class $ζ\in H^n(G,k)$ is called productive if it annihilates $\Ext^*_{kG}(L_ζ,L_ζ)$. We consider the chain complex $\bPz$ of projective $kG$-modules which has the homology of an $(n-1)$-sphere and whose $k$-invariant is $ζ$ under a certain polarization. We show that $ζ$ is productive if and only if there is a chain map $Δ: \bPz \to \bPz \otimes \bPz$ such that $(\id \otimes ε)Δ\simeq \id$ and $(ε\otimes \id)Δ\simeq \id$. Using the Postnikov decomposition of $\bPz \otimes \bPz$, we prove that there is a unique obstruction for constructing a chain map $Δ$ satisfying these properties. Studying this obstruction more closely, we obtain theorems of Carlson and Langer on productive elements.

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arXiv2012

Fusion systems and group actions with abelian isotropy subgroups

We prove that if a finite group $G$ acts smoothly on a manifold $M$ so that all the isotropy subgroups are abelian groups with rank $\leq k$, then $G$ acts freely and smoothly on $M \times \bbS^{n_1} \times...\times \bbS^{n_k}$ for some positive integers $n_1,...n_k$. We construct these actions using a recursive method, introduced in an earlier paper, that involves abstract fusion systems on finite groups. As another application of this method, we prove that every finite solvable group acts freely and smoothly on some product of spheres with trivial action on homology.

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arXiv2012

Constructing homologically trivial actions on products of spheres

We prove that if a finite group $G$ has a representation with fixity $f$, then it acts freely and homologically trivially on a finite CW-complex homotopy equivalent to a product of $f+1$ spheres. This shows, in particular, that every finite group acts freely and homologically trivially on some finite CW-complex homotopy equivalent to a product of spheres.

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arXiv2010

Acyclic Chain complexes over the Orbit Category

Chain complexes of finitely generated free modules over orbit categories provide natural algebraic models for finite G-CW complexes with prescribed isotropy. We prove a p-hypoelementary Dress induction theorem for K-theory over the orbit category of a finite group, and use it to re-interpret some results of Oliver and Kropholler-Wall on acyclic complexes.

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arXiv2010

Fusion systems and constructing free actions on products of spheres

We show that every rank two $p$-group acts freely and smoothly on a product of two spheres. This follows from a more general construction: given a smooth action of a finite group $G$ on a manifold $M$, we construct a smooth free action on $M \times \bbS ^{n_1} \times \dots \times \bbS ^{n_k}$ when the set of isotropy subgroups of the $G$-action on $M$ can be associated to a fusion system satisfying certain properties. Another consequence of this construction is that if $G$ is an (almost) extra-special $p$-group of rank $r$, then it acts freely and smoothly on a product of $r$ spheres.

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arXiv2010

Equivariant CW-Complexes and the Orbit Category

We give a general framework for studying G-CW complexes via the orbit category. As an application we show that the symmetric group G=S_5 admits a finite G-CW complex X homotopy equivalent to a sphere, with cyclic isotropy subgroups.

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arXiv2006

Quadratic Maps and Bockstein Closed Group Extensions

We study central extensions E of elementary abelian 2-groups by elementary abelian 2-groups. Associated to such an extension is a quadratic map which determines the extension uniquely. The components of the map determine a quadratic ideal in a polynomial algebra and we say that the extension is Bockstein closed if this ideal is invariant under the Bockstein operator. We find a direct condition on the quadratic map that characterizes when the extension is Bockstein closed. Using this we show for example that quadratic maps induced from the fundamental quadratic map on gl_n, Q(A)=A+A^2 yield Bockstein closed extensions. We also show that this condition is equivalent to a certain liftability of the extension and under certain conditions, to the fact that Q corresponds to a 2-power map of a restricted 2-Lie algebra. For the class of 2-groups coming from these type of extensions, under a 2-power exact condition we also compute the mod 2 cohomology ring of the corresponding groups - this includes the upper triangular congruence subgroups among other examples.

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arXiv2006

Linear colorings of simplicial complexes and collapsing

A vertex coloring of a simplicial complex $Δ$ is called a linear coloring if it satisfies the property that for every pair of facets $(F_1, F_2)$ of $Δ$, there exists no pair of vertices $(v_1, v_2)$ with the same color such that $v_1\in F_1\backslash F_2$ and $v_2\in F_2\backslash F_1$. We show that every simplicial complex $Δ$ which is linearly colored with $k$ colors includes a subcomplex $Δ'$ with $k$ vertices such that $Δ'$ is a strong deformation retract of $Δ$. We also prove that this deformation is a nonevasive reduction, in particular, a collapsing.

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