Alejandro Adem


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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

On a completion of cohomological functors generalizing Tate cohomology (2024)

Viewing group cohomology as a so-called cohomological functor, G. Mislin has generalized Tate cohomology from finite groups to all discrete groups by defining a completion for cohomological functors in his paper "Tate Cohomology for Arbitrary Groups via Satellites". We construct a Mislin completion for any cohomological functor whose domain category is an abelian category with enough projectives and whose codomain category is an abelian category in which all countable direct limits exist and are exact. This takes Tate cohomology to settings where it has never been introduced such as in condensed mathematics. Through the latter, one can define Tate cohomology for any topological group all whose points are closed. More specifically, we generalize four constructions of Mislin completions from the literature, prove that they yield isomorphic cohomological functors and provide explicit formulae for their connecting homomorphisms. For any morphism in the domain category we develop formulae for the induced morphism in each degree of the Mislin completion in terms of each construction. As their main feature, Mislin completions of Ext-functors detect finite projective dimension of objects in the domain category. Moreover, we establish a version of dimension shifting, an Eckmann-Shapiro result as well as Yoneda and external products.

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Classifying space for commutativity and unordered flag manifolds (2023)

The first part of this thesis is dedicated to the computation of the cohomology of the total space of the principal U(3)-bundle of the classifying space for commutativity for U(3). The second part delves into examining the stable and unstable cohomology rings of the unordered flag manifolds.Classifying space for commutativity in U(3).The total space of the principal G-bundle associated with the classifying space for commutativity BcomG is denoted by EcomG. In Chapter 2, we describe EcomU(3) as a homotopy colimit of a diagram of spaces and detail a method of computation of the mod p cohomology of EcomU(3) by utilizing the spectral sequence associated with a homotopy colimit. In order to perform this computation, it is important to determine the cohomology of various spaces that are present in the homotopy colimit diagram. We present some of these computations, which are fascinating on their own, and delve into intriguing topics that we explore further in Chapters 3 and 4. We also present the ring structure of the rational cohomology of EcomU(3).Cohomology of the unordered flag manifolds.Unordered flag manifolds are the orbit spaces of the natural action of the symmetric group on the complete flag manifolds. The complex unordered flag manifold of order n can be defined as the total space of a fiber sequence, wherein the base is the extended power of U(1), and the fiber is U(n). In Chapter 3, we establish the Hopf ring structure of the cohomology of extended power of a space. Utilizing this description, we prove the homological stability of the complex unordered flag manifolds and describe their stable cohomology. In Chapter 4, we demonstrate a pullback formula and present an algorithmic approach to computing the unstable cohomology of the unordered flag manifolds using spectral sequences. We also detail a few low-dimensional cohomology computations with mod 2 coefficients and offer an infinite family of examples, namely the mod p cohomology of the unordered flag manifolds of order p.

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Decomposition of topological Azumaya algebras in the stable range (2021)

In this thesis, we establish decomposition theorems for topological Azumaya algebras, and topological Azumaya algebras with involutions of the first kind. Decomposition of topological Azumaya algebrasLet A be a topological Azumaya algebra of degree mn over a CW complex X. We prove that if m and n are relatively prime, m
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Applications and connections between twisted equivariant K-theory, quantum mechanics and condensed matter (2020)

The present thesis consists of 2 parts. Chapter 1 is about applications of twisted equivariant K-theory to condensed matter. We consider non-interacting electrons on a half-crystal (a crystal with a boundary), with a gapped bulk condition, under quasi-adiabatic evolution. In A. Adem, O. Antolin, G. Semenoff and D. Sheinbaum JHEP, 2016 we found that Fermi surfaces for these systems under quasi-adiabatic evolution are classified by the K⁻¹-group of the surface Brillouin zone Td⁻¹. Systems with time-reversal and particle-hole symmetry were also considered and we obtained different KR-groups for the different cases. In Chapter 1 I rewrite A. Adem, O. Antolin, G. Semenoff and D. Sheinbaum JHEP, 2016 in a more function-analytic language and further solve technical issues to extend it to include crystallographic symmetries on the directions parallel to the boundary. In Chapter 2 I reproduce the relevant parts of my joint work with C. Okay (C. Okay and D. Sheinbaum arXiv:1905.07723). There we explored a connection between twisted equivariant K-theory to contextuality in quantum mechanics. We also reformulated the sheaf-theoretic framework of S. Abramsky and A. Brandenburger New Journal of Physics, 2011 for contextuality and connect it to another one employing a group cohomology approach of C. Okay, S. Roberts, S.D Bartlett, and R. Raussendorf Quantum Information and Computation, 2017. This leads to the construction of a classifying space for contextuality, from which Wigner functions are classes in its twisted K-theory.

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A simplicial approach to spaces of homomorphisms (2017)

Let G be a real linear algebraic group and L a finitely generated cosimplicial group. We prove that the space of homomorphisms Hom(Ln;G) has a homotopy stable decomposition for each n ≥ 1. When G is a compact Lie group, we show that the decomposition is G-equivariant with respect to the induced action of conjugation by elements of G. In particular, under these hypotheses on G, we obtain stable decompositions for Hom(Fqn;G) and Rep(Fqn;G) respectively, where Fqn are the finitely generated free nilpotent groups of nilpotency class q-1. The spaces Hom(Ln;G) assemble into a simplicial space Hom(L;G). When G=U we show that its geometric realization B(L;U) has a non-unital E-infinity-ring space structure whenever Hom(L0;U(m)) is path connected for all m ≥ 0.We describe the connected components of Hom(Fqn;SU(2)) arising fromnon-commuting q-nilpotent n-tuples. We prove this by showing that all these n-tuples are conjugated to n-tuples consisting of elements in the the generalizedquaternion groups Q2q in SU(2), of order 2^q. Using this result, we exhibit thehomotopy type of SHom(Fqn;SU(2)) and a homotopy description of the classifying spaces B(q;SU(2)) of transitionally q-nilpotent principal SU(2)-bundles. The above computations are also done for SO(3) and U(2).Finally, for q = 2, the space B(2;G) is denoted BcomG, and we compute theintegral cohomology ring for the Lie groups G = SU(2) andU(2). We also includecohomology calculations for the spaces BcomQ2q .

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Representation Rings of Semidirect Products of Tori by Finite Groups (2015)

This dissertation studies semidirect products of a torus by a finite group from the representation theory point of view. The finite group of greatest interest is the cyclic group of prime order. Such semidirect products occur in nature as isotropy groups of Lie groups acting on themselves by conjugation and as normalizers of maximal tori in reductive linear algebraic groups. The main results of this dissertation are: a) the calculation of the representation ring of such semidirect products as an algebra over the integers for certain special cases, b) the adaptation of an algorithm from invariant theory to find finite presentations of representation rings, c) the computation of the topological K-theory of the classifying space of certain semidirect products, d) the demonstration that the equivariant K-theory of the projective unitary group of degree 2 acting on itself by conjugation is not a free module over its representation ring.

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Homotopy colimits of classifying spaces of finite abelian groups (2014)

The classifying space BG of a topological group G can be filtered by a sequence of subspaces B(q,G) using the descending central series of free groups. If G is finite, describing them as homotopy colimits is convenient when applying homotopy theoretic methods. In this thesis we introduce natural subspaces B(q,G)_p of B(q,G) defined for a fixed prime p. We show that B(q,G) is stably homotopy equivalent to a wedge sum of B(q,G)_p as p runs over the primes dividing the order of G. Colimits of abelian groups play an important role in understanding the homotopy type of these spaces. Extraspecial p-groups are key examples, for which these colimits turn out to be finite. We prove that for extraspecial p-groups of rank at least 4 the space B(2,G) does not have the homotopy type of a K(π,1) space. Furthermore, we give a group theoretic condition, applicable to symmetric groups and general linear groups, which implies the space B(2,G) not having the homotopy type of a K(π,1) space. For a finite group G, we compute the complex K-theory of B(2,G) modulo torsion.

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Spaces of Homomorphisms and Commuting Orthogonal Matrices (2014)

In this work we study the space of group homomorphisms Hom (π,G) forspecial choices of π and G. In the first part of this thesis, we enumerate anddescribe the path components for the spaces of ordered commuting k-tuplesof orthogonal and special orthogonal matrices respectively. This correspondsto choosing π = ℤk and G = O(n); SO(n). We also provide a lower boundon the number of components for the case G = Spin(n) for suffciently largen. In the second part, we describe the space Hom (Г,SU(2)), where Г is agroup arising from a central extension of the form0 → ℤr → Г → ℤk → 0.The description of this space is good enough that, using some known results,it allows us to compute its cohomology groups.

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Group actions on homotopy spheres (2011)

In the first part of the thesis we discuss the rank conjecture of Benson and Carlson. In particular, we prove that if G is a finite p-group of rank 3 and with p odd, or if G is a central extension of abelian p-groups, then there is a free finite G-CW-complex homotopy equivalent to the product of rk(G) spheres; where rk(G) is the rank of G.We also treat an extension of the rank conjecture to groups of finite virtual cohomological dimension. In this context, for p a fixed odd prime, we show that there is an infinite group L satisfying the two following properties: every finite subgroup G
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Fusion Algebras and Cohomology of Toroidal Orbifolds (2010)

In this thesis we exhibit an explicit non-trivial example of the twisted fusion algebra for a particular finite group. The product is defined for the third power of modulo two group via the pairing of projective representations where the three cocycles are chosen using the inverse transgression map. We find the rank of the fusion algebra as well as the relation between its basis elements. We also give some applications to topological gauge theories. We next show that the twisted fusion algebra of the third power of modulo p group is isomorphic to the non-twisted fusion algebra of the extraspecial p-group of order p³ and exponent p.The final point of my thesis is to explicitly compute the cohomology groups of toroidal orbifolds which are the quotient spaces obtained by the action of modulo p group on the k-dimensional torus. We compute the particular case where the action is induced by the n-th power of augmentation ideal.

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Equivariant K-theory, groupoids and proper actions (2009)

Equivariant K-theory for actions of groupoids is defined and shown to bea cohomology theory on the category of finite equivariant CW-complexes.Under some conditions, these theories are representable. We use this fact todefine twisted equivariant K-theory for actions of groupoids. A classificationof possible twistings is given. We also prove a completion theorem for twistedand untwisted equivariant K-theory. Finally, some applications to proper actions of Lie groups are discussed.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Jones-cosmetic tangle replacement (2023)

Bar-Natan observed that the knots 5₁ and 10₁₃₂ have identical Jones polynomial, while recent work of Baldwin, Hu and Sivek shows that the cinquefoil 5₁ is detected by Khovanov homology. These two knots are related by a Jones-cosmetic tangle replacement, under which the (3,-2) pretzel tangle found within 10₁₃₂ is replaced by a rational tangle. The theory of immersed curves developed by Kotelskiy, Watson and Zibrowius provides us with a combinatorial means of computing reduced Bar-Natan homology, via which we investigate the existence and uniqueness of Jones-cosmetic pairs formed of a two-bridge knot and a rational tangle closure of the (3,-2) pretzel tangle with determinant less than or equal to 5. Using the observation that Jones polynomials with different spans are different, we prove that there does not exist a Jones-cosmetic pair associated with the (3,-2) pretzel tangle involving the unknot or the trefoil. Moreover, we prove that 5₁ and 10₁₃₂form the unique Jones-cosmetic pair associated with the (3,-2) pretzel tangle with determinant equal to 5.

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Topological methods of preference and judgment aggregation (2011)

Arrow’s Impossibility Theorem is a classical result in social choice theory (a branch of economic theory), which states that any system of rules for combining (“aggregating”) individual preference relations into a single representative relation results in a “dictatorship” where the combined preference only reflects the wishes of a single individual (provided that the aggregation rule satisfies two basic criteria). Since the 1980s, this result has been reformulated and understood using algebraic topology. The topological approach offers some geometric intuition as to why Arrow’s theorem holds, and can also be used to find alternative hypotheses which may escape the dictatorship outcome. A thorough examination of such topological models constitutes the main body of this thesis.Recently, social choice theory has been generalized (resulting in a field called “judgment aggregation”), and results analogous to Arrow's theorem have been established. The second part of this thesis introduces this field of study, and studies how some of the techniques from topological social choice theory can be extended to understand dictatorship outcomes in the theory of judgment aggregation. Although the analysis is restricted to a rather simple case, it nonetheless highlights the potential for a more general topological model of judgment aggregation, and exposes the main challenges that must be overcome in constructing such a theory.

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