Events - Page 2

Time and place: Mar. 20, 2018 10:15 AM12:00 PM, End of the Line, Sognsveien 77 B

This talk is supposed to be an Introductionary talk to the preprint arXiv:1409.4372v4 (joint work with G.Garkusha). More specifically, using the theory of framed correspondences developed by Voevodsky, the authors introduce and study framed motives of algebraic varieties. This study gives rise to a construction of the big frame motive functor. It is shown that this functor converts the classical Morel--Voevodsky motivic stable homotopy theory into an equivalent local theory of framed bispectra, and thus producing a new approach to stable motivic homotopy theory. As a topological application, it is proved that for the simplicial set Fr(Delta^\bullet_C, S^1) has the homotopy type of the space \Omega^{\infty} Sigma^{\infty} (S^1). Here C is the field complex numbers. 

Time and place: Mar. 8, 2018 10:15 AM2:00 PM, Desolation Row Sognsveien 77 B

I discuss how Bökstedt and Madsen (1994/1995) calculate mod p homotopy for THH(Z) and the fixed-point spectra THH(Z)^{C_{p^n}}, together with the R- and F-operators. This leads to a calculation for TC(Z; p) and K(Z_p), confirming the Lichtenbaum-Quillen conjecture in this case. 

Time and place: Mar. 7, 2018 2:15 PM4:00 PM, Desolation Row, Sognsveien 77 B

I will review Bökstedt, Hesselholt and Madsen's calculations of the topological cyclic homology of prime fields and the integers, again taking into account simplifications made in later papers. (If necessary, I will continue on Thursday.)   

Time and place: Mar. 7, 2018 10:15 AM12:00 PM, Desolation Row, Sognsveien 77 B
Time and place: Mar. 6, 2018 10:15 AM12:00 PM, End of the Line, Sognsveien 77B

Recently two different refinements of Voevodsky's theory of presheaves with transfers were introduced: the first one is the theory of framed presheaves based on the unpublished notes by Voevodsky and developed by Garkusha and Panin and the second one is the theory of Milnor-Witt presheaves due to Calmes and Fasel. I will review some relations between these theories and explain that the hearts of the homotopy t-structures on the corresponding categories of motives are naturally equivalent. The talk is based on a joint work with A. Neshitov. 

Time and place: Feb. 28, 2018 2:15 PM4:00 PM, Desolation Row Sognsveien 77B

In this third talk we will define Legendrian contact homology for Legendrian submanifolds in the 1-jet space of a smooth manifold M. Again, this will be the homology of a DGA generated by the double points of the Legendrian under the Lagrangian projection. The differential is defined by a count of punctured pseudo-holomorphic disks in the cotangent bundle of M, with boundary on the projected Legendrian. To prove that this indeed gives a differential we will use the theory of Fredholm operators from functional analysis. I will also say something about Floer theories in general. In particular, one of the main difficulties when defining Floer theories via pseudo-holomorphic curve techniques is to achieve transversality for the dbar-operator. There has been a development of several different machineries to solve these problems, for examle Polyfolds by Hofer et al., and Pardon's work on Virtual fundamental cycles. In our case, however, it is enough to perturb either the Legendrian submanifold or the almost complex structure.   

Time and place: Feb. 28, 2018 10:15 AM12:00 PM, Desolation Row Sognsveien 77B
Time and place: Feb. 21, 2018 2:15 PM4:00 PM, Desolation Row, Sognsveien 77B

I will review Marcel Bökstedt's calculation of the topological Hochschild homology of prime fields and the integers, taking into account simplifications made in papers by Angeltveit-R. (where BP<m-1> specializes to HFp for m=0 and to HZ(p) for m=1) and Ausoni (proof of Lemma 5.3).

Time and place: Feb. 21, 2018 10:15 AM12:00 PM, Desolation Row Sognsveien 77 B

Inspired by the Voevodsky machinery of standard triples a machinery of nice triples was invented in [PSV]. We develop further the latter machiny such that it works also in the finite field case [P]. This machinary is a tool to prove many interesting moving lemmas. It leads to a serios of applications. One of them is a proof of the Grothendieck--Serre conjecture in the finite field case. Another is a proof of Gersten type results for arbitrary cohomology theories on algebraic varieties. The Gersen type results allows to conclude the following: a presheaf of S1-spectra E on the category of k-smooth schemes is A1-local iff all its Nisnevich sheaves of stable A1-homotopy groups are strictly homotopy invariant. If the field k is infinite, then the latter result is due to Morel [M]. An example of moving lemma is this. Let X be a k-smooth quasi-projective irreducible k-variety, Z be its closed subset and x be a finite subset of closed points in X. Then there exists a Zariski open U containing x and a naive A1-homotopy between the motivic space morphism U--> X--> X/U and the morphism U--> X/U sending U to the distinguished point of X/U. Application: suppose E is a cohomology theory on k-smooth varieties and alpha is an E-cohomology class on X which vanishes on the complement of Z, then it vanishes on U from the lemma above.   

Time and place: Feb. 14, 2018 2:15 PM4:00 PM, Desolation Row Sognsveien 77B

In this second talk, I will define Chekanov's version of Legendrian contact homology (LCH) for Legendrian knots in R3. I will begin with an example, showing that LCH is more sensitive than the classical invariants. This will use a linearized version of the homology. In the second part of the talk I will focus on the proof that the differential indeed squares to zero, and also say something about invariance under Legendrian Reidemeister moves. This is intended to be a smooth introduction to the next talk, where we will consider Legendrian contact homology defined for Legendrians in arbitrary 1-jet spaces. This case is more delicate, and we have to understand the concept of Gromov compactness for pseudo-holomorphic curves to prove that we get a differential graded algebra associated to each Legendrian, whose homology will give a Legendrian invariant.

Time and place: Feb. 14, 2018 10:15 AM12:00 PM, Desolation Row Sognsveien 77B

Let G be a finite (abstract) group and let k be a field of characteristic zero. We prove that for a non-singular projective G-variety X over k, and a non-singular G-invariant subvariety Y of dimension >= 3, which is a scheme-theoretic complete intersection in X, the pullback map PicG(X) -> PicG(Y) is an isomorphism. This is an equivariant analog of the Grothendieck-Lefschetz theorem for Picard groups.   

Time and place: Feb. 7, 2018 2:15 PM4:00 PM, Desolation Row Sognsveien 77B

A Cartan-Eilenberg system is an algebraic structure introduced as a model of the diagram obtained by taking the homology of all subquotients in a filtered chain complex. There are two exact couples and a single spectral sequence associated with such a system, and one may thus apply Boardman's theory of convergence to either exact couple. After reviewing parts of this theory, I will clarify the convergence situation in a Cartan-Eilenberg system and in particular present new work on a simpler interpretation of Boardman's whole plane obstruction group.   

Time and place: Jan. 31, 2018 2:15 PM4:00 PM, Desolation Row, Sognsveien 77 B

I will give a series of talks about Legendrian contact homology, an invariant of Legendrian submanifolds in 1-jet spaces, defined by a count of pseudo-holomorphic curves. In this first lecture I will give a brief and gentle introduction to symplectic and contact geometry, with focus on Lagrangian and Legendrian submanifolds. No previous knowledge about the subject is needed, except for elementary knowledge about differentiable manifolds.   

Time and place: Nov. 22, 2017 2:15 PM4:00 PM, Desolation Row, Sognsveien 77 B

I will discuss the differential structure in the mod 2 Adams spectral sequence for tmf, leading to its E_\infty-term.  These calculations were known to Hopkins-Mahowald; in their current guise they are part of joint work with Bruner.

Time and place: Nov. 15, 2017 2:30 PM4:00 PM, Desolation Row, Sognveien 77 B

I will report on work in progress on calculations of the motivic homotopy groups of MGL (the algebraic cobordism spectrum) over number fields. It is known that pi_{2n,n}(MGL) is the Lazard ring, and pi_{-n,-n}(MGL) is Milnor K-theory of the base field. We will calculate all of pi_{*,*}(MGL) with the slice spectral sequence (motivic Atiyah-Hirzebruch spectral sequence) over a number field. I will give a brief review of the the tools and sketch the main parts of the calculation: The input from motivic cohomology, the use of C_2-equivariant Betti realization and comparison with Hill-Hopkins-Ravenel to determine the differentials, and settle most of the hidden extensions. 

Time and place: Nov. 8, 2017 3:00 PM4:00 PM, Hurricane, Sognsveien 77 b

I will discuss the algebra structure of the E_2-term of the mod 2 Adams spectral sequence for tmf, given by the cohomology Ext_{A(2)}(F_2, F_2) of A(2).  We (Bruner & Rognes) use Groebner bases to verify the presentation given by Iwai and Shimada, with 13 generators and 54 relations. Thereafter I will discuss the relationship between differentials and Steenrod operations in the Adams spectral sequence for E_\infty ring spectra.  

Time and place: Nov. 1, 2017 2:15 PM4:00 PM, Desolation Row, Sognsveien 77 B

I will discuss machine computations in a finite range, using Bruner's ext-program, of Ext over A, the mod 2 Steenrod algebra, and over A(2), the subalgebra of A generated by Sq^1, Sq^2 and Sq^4. These are the E_2-terms of the mod 2 Adams spectral sequences for S and tmf, respectively.

Time and place: May 10, 2017 2:15 PM4:00 PM, B 738

Topological cyclic homology is a variant of negative cyclic homology which was introduced by Bökstedt, Hsiang and Madsen. They invented topological cyclic homology to study algebraic K-theory but in recent years it has become more and more important as an invariant in its own right. We present a new formula for topological cyclic homology and give an entirely model independent construction. If time permits we explain consequences and further directions.

Time and place: May 3, 2017 2:15 PM4:00 PM, B 738

Joint work with Bjørn I. Dundas. We prove that algebraic K-theory, topological Hochschild homology and topological cyclic homology satisfy cubical and cosimplicial descent at connective structured ring spectra along 1-connected maps of such ring spectra.

Time and place: Apr. 26, 2017 2:15 PM4:00 PM, B 738

 In this talk all spaces and spectra will be localised at 2. Many E-infinity ring spectra turn out to be `finitely generated' in the sense that there is finite CW spectrum and a map from the free E-infinity ring spectrum generated by it inducing an epimorphism in mod 2 homology. This turns out to be an interesting condition and I will discuss some examples such as HZ, kO, kU, tmf and tmf_1(3). One long term goal of this work is to produce `ultra-generalised Brown-Gitler spectra' and I will discuss this idea if there is time.

Time and place: Apr. 3, 2017 2:15 PM4:00 PM, B 637

Given a knot K in the 3-sphere, we use Heegaard Floer correction terms to give lower bounds on the first Betti number of (orientable and non-orientable) surfaces in the 4-ball with boundary K. An amusing feature of the non-orientable bound is its superadditivity with respect to connected sums. This is joint work with Marco Marengon. If time permits, I will discuss relations with deformations of singularities of curves (joint work with József Bodnár and Daniele Celoria). 

Time and place: Mar. 15, 2017 2:15 PM4:00 PM, B 738

In the 80's Bökstedt introduced THH(A), the Topological Hochschild homology of a ring A, and a trace map from algebraic K-theory of A to THH(A). This trace map, along with the circle action on THH, have since been used extensively to make calculations of algebraic K-theory. When the ring A has an anti-involution Hesselholt and Madsen have promoted the spectrum K(A) to a genuine Z/2-spectrum whose fixed points is the K-theory of Hermitian forms over A. They also introduced Real topological Hochschild homology THR(A), which is a genuine equivariant refinement of THH, and Dotto constructed an equivariant refinement of Bökstedt's trace map. I will report on recent joint work with Dotto, Patchkoria and Reeh on models for the spectrum THR(A) and calculations of its RO(Z/2)-graded homotopy groups. 

Time and place: Feb. 22, 2017 2:15 PM4:00 PM, B 738

The classical s-cobordism theorem classifies completely h-cobordisms from a fixed manifold, but it does not tell us much about the relationship between the two ends. In the talk I will present some old and new results about this. I will also discuss how this relates to a seemingly different problem: what can we say abobut two compact manifolds M and N if we know that MxR and NxR are diffeomorphic? This is joint work with Slawomir Kwasik, Tulane, and Jean-Claude Hausmann, Geneva.

Time and place: Feb. 22, 2017 12:15 PM2:00 PM, B 637

I will survey the connection between the space H(M) of h-cobordisms on a given manifold M, several categories of spaces containing M, Waldhausens algebraic K-theory A(M), and the algebraic K-theory of the suspension ring spectrum S[?M] of the loop space of M. The results extend the h-cobordism theorem of Smale and the s-cobordism theorem of Barden, Mazur and Stallings to a parametrized h-cobordism theorem, valid in a stable range established by Igusa, first discussed by Hatcher and finally proved and published by Waldhausen, Jahren and myself.

Time and place: Feb. 1, 2017 2:15 PM4:00 PM, B 738

The Barratt nerve BSd X of the Kan subdivision Sd X of a simplicial set X \in sSet is a triangulation. The Barratt nerve is defined as taking the poset of non-degenerate simplices, thinking of it as a small category and then finally taking the nerve.Waldhausen, Jahren and Rognes (Piecewise linear manifolds and categories of simple maps) named this construction 'the improvement functor' because of the homotopical properties and because its target is non-singular simplicial sets. A simplicial set is said to be 'non-singular' if its non-degenerate simplices are embedded. There is a least drastic way of making a simplicial set non-singular called 'desingularization', which is a functor D:sSet -> nsSet that is left adjoint to the inclusion. The functor DSd^2 is the left Quillen functor of a Quillen equivalence where the model structure on sSet is the standard one where the weak equivalences are those that induce weak homotopy equivalences and the fibrations are the Kan fibrations. I will talk about the main steps of the proof that the natural map DSd X -> BX is an isomorphism for regular X. This implies that DSd^2 is a triangulation and that the improvement functor is less ad hoc than it may seem. Furthermore, I will explain how the result provides evidence that any cofibrant non-singular simplicial set is the nerve of some poset.