Analytic Detection in Homotopy Groups of Smooth Manifolds

In this paper, for the mapping of a sphere into a compact orientable manifoldSnM,n1, we solve the problem of determining whether it represents a nontrivial element in the homotopy group of the manifoldn(M) n(M ). For this purpose, we consistently use the theory of iterated integrals developed by K.-T. Chen. It should be noted that the iterated integrals as repeated integration were previously meaningfully used by Lappo-Danilevsky to represent solutions of systems of linear differential equations and by Whitehead for the analytical description of the Hopf invariant for mappingsf:S2n-1Sn,n2. We give a brief description of Chens theory, representing Whiteheads and Haefligers formulas for the Hopf invariant and generalized Hopf invariant. Examples of calculating these invariants using the technique of iterated integrals are given. Further, it is shown how one can detect any element of the fundamental group of a Riemann surface using iterated integrals of holomorphic forms. This required to prove that the intersection of the terms of the lower central series of the fundamental group of a Riemann surface is a unit group.

Residual dimension of nilpotent groups

The function {\mathrm{F}_{G}(n)} gives the maximum order of a finite group needed to distinguish a nontrivial element of G from the identity with a surjective group morphism as one varies over nontrivial elements of word length at most n. In previous work [M. Pengitore, Effective separability of finitely generated nilpotent groups, New York J. Math. 24 2018, 83–145], the author claimed a characterization for {\mathrm{F}_{N}(n)} when N is a finitely generated nilpotent group. However, a counterexample to the above claim was communicated to the author, and consequently, the statement of the asymptotic characterization of {\mathrm{F}_{N}(n)} is incorrect. In this article, we introduce new tools to provide lower asymptotic bounds for {\mathrm{F}_{N}(n)} when N is a finitely generated nilpotent group. Moreover, we introduce a class of finitely generated nilpotent groups for which the upper bound of the above article can be improved. Finally, we construct a class of finitely generated nilpotent groups N for which the asymptotic behavior of {\mathrm{F}_{N}(n)} can be fully characterized.

Groups Acting on Trees

This chapter considers groups acting on trees. It examines which groups act on which spaces and, if a group does act on a space, what it says about the group. These spaces are called trees—that is, connected graphs without cycles. A group action on a tree is free if no nontrivial element of the group preserves any vertex or any edge of the tree. The chapter first presents the theorem stating that: If a group G acts freely on a tree, then G is a free group. The condition that G is free is equivalent to the condition that G acts freely on a tree. The discussion then turns to the Farey tree and shows how to construct the Farey complex using the Farey graph. The chapter concludes by describing free and non-free actions on trees. Exercises and research projects are included.

Non-commutative lattice problems

We consider several subgroup-related algorithmic questions in groups, modeled after the classic computational lattice problems, and study their computational complexity. We find polynomial time solutions to problems like finding a subgroup element closest to a given group element, or finding a shortest nontrivial element of a subgroup in the case of nilpotent groups, and a large class of surface groups and Coxeter groups. We also provide polynomial time algorithm to compute geodesics in given generators of a subgroup of a free group.

Bott periodicity in the Hit Problem

In this short paper, we use Robert Bruner's $\cal{A}$(1)-resolution of $P = {\mathbb{F}_2[t]$ to shed light on the Hit Problem. In particular, the reduced syzygies Pn of P occur as direct summands of $\widetilde{P}^{\otimes n}$, where $\widetilde{P}$ is the augmentation ideal of the map $P \to \mathbb{F}_2$. The complement of Pn in $\widetilde{P}^{\otimes n}$ is free, and the modules Pn exhibit a type of “Bott periodicity” of period 4: Pn+4 = Σ8Pn. These facts taken together allow one to analyse the module of indecomposables in $\widetilde{P}^{\otimes n}$, that is, to say something about the “$\cal{A}$(1)-hit Problem”. Our study is essentially in two parts: first, we expound on the approach to the Hit Problem begun by William Singer, in which we compare images of Steenrod squares to certain kernels of squares. Using this approach, the author discovered a nontrivial element in bidegree (5, 9) that is neither $\cal{A}$(1)-hit nor in kerSq1 + kerSq3. Such an element is extremely rare, but Bruner's result shows clearly why these elements exist and detects them in full generality; second, we describe the graded ${\mathbb{F}_2$-space of $\cal{A}$(1)-hit elements of $\widetilde{P}^{\otimes n}$ by determining its Hilbert series.

THE BRAUER GROUP OF DEL PEZZO SURFACES

Let S1 be a Del Pezzo surface of degree 1 over a number field k. We establish a criterion for the existence of a nontrivial element of order 5 in the Brauer group of S1 in terms of certain Galois-stable configurations of exceptional divisors on this surface.

Detection of a new nontrivial family in the stable homotopy of spheres $ \mbf{\pi_{\ast}S} $

To determine the stable homotopy groups of spheres is one of the central problems in homotopy theory. Let $ A $ be the mod $ p $ Steenrod algebra and $S$ the sphere spectrum localized at an odd prime $ p $. In this article, it is proved that for $ p\geqslant 7 $, $ n\geqslant 4 $ and $ 3\leqslant s $, $ b_0 h_1 h_n \tilde{\gamma}_{s} \in Ext_A^{s+4,\ast}(\mathbb{Z}_p,\mathbb{Z}_p) $ is a permanent cycle in the Adams spectral sequence and converges to a nontrivial element of order $ p $ in the stable homotopy groups of spheres $ \pi_{p^nq+sp^{2}q+(s+1)pq+(s-2)q-7}S $, where $ q=2(p-1 ) $.

Symmetries of Kirchberg Algebras

Let G0 and G1 be countable abelian groups. Let γi be an automorphism of Gi of order two. Then there exists a unital Kirchberg algebra A satisfying the Universal Coefficient Theorem and with [1A] = 0 in K0(A), and an automorphism α ∈ Aut(A) of order two, such that K0(A) ≅ G0, such that K1(A) ≅ G1, and such that α* : Ki(A) → Ki(A) is γi. As a consequence, we prove that every -graded countable module over the representation ring R() of is isomorphic to the equivariant K-theory K (A) for some action of on a unital Kirchberg algebra A.Along the way, we prove that every not necessarily finitely generated []-module which is free as a -module has a direct sum decomposition with only three kinds of summands, namely [] itself and on which the nontrivial element of acts either trivially or by multiplication by −1.

On centralizers of elements of groups acting on trees with inversions

A subgroupHof a groupGis called malnormal inGif it satisfies the condition that ifg∈Gandh∈H,h≠1such thatghg−1∈H, theng∈H. In this paper, we show that ifGis a group acting on a treeXwith inversions such that each edge stabilizer is malnormal inG, then the centralizerC(g)of each nontrivial elementgofGis in a vertex stabilizer ifgis in that vertex stabilizer. Ifgis not in any vertex stabilizer, thenC(g)is an infinite cyclic ifgdoes not transfer an edge ofXto its inverse. Otherwise,C(g)is a finite cyclic of order 2.