- 09/12/2019: Topology in Special Holonomy
- 06/07/2019: Gluing constructions for associatives
- 01/08/2019: Building blocks for twisted connected sums
- 01/09/2019: The matching problem for twisted connected sums
- 01/12/2018: G
_{2}-instantons on twisted connected sums - 09/11/2017: New asymptotically conical G
_{2}-manifolds - 06/05/2017: Twisted connected sum G
_{2}-manifolds - 06/06/2017: Topological invariants of G
_{2}-manifolds - 09/06/2016: Complete and conically singular G
_{2}-manifolds of cohomogeneity one

### September 12, 2019

TITLE: Topology in Special Holonomy

ABSTRACT: Nordstrom will discuss what scant information has been available about obstructions to the existence of G2-holonomy metrics on closed 7-manifolds and then go on to explain some recent progress on the topology of known and recently constructed examples. Many of these 7-manifolds belong to classes where diffeomorphism classification results have been proved or are within reach, making it possible to map out a portion of the landscape of closed G2-manifolds. In particular, we can exhibit phenomena such as homeomorphic, but not diffeomorphic, examples and the non-connectedness of the moduli space of G2-metrics on specific 7-manifolds.

Slides of lecture

### June 7, 2019

TITLE: Gluing constructions for associatives

ABSTRACT: I will discuss two related gluing constructions of associative submanifolds that I have been promising to complete for a long time. One is to construct associative 3-spheres in many twisted connected sum G_{2}-manifolds as analogously twisted connected sums of complex lines. The other is to resolve self-intersection singularities of immersed associatives by gluing in Lawlor necks. If the immersed associative is unobstructed, then it will deform uniquely under small deformations of the ambient G_{2}-structure. If the self-intersection is transverse in a 1-parameter family of G_{2}-structures (so in particular, the immersed associative fails to be embedded at a single parameter t_{0}) this yields a second family of associatives that is created/destroyed at t_{0}, as envisaged by e.g. Donaldson and Joyce.

### January 8, 2019

TITLE: Building blocks for twisted connected sums

ABSTRACT: The twisted connected sum method of Kovalev and Corti-Haskins-Nordström-Pacini allows the construction of large numbers of closed manifolds with holonomy . The “building blocks” of the construction are a certain kind of closed Kähler 3-folds. The aim of the talk is to review the basics of the construction (and the “extra-twisted” version of Crowley-Goette-Nordström), and to discuss some algebraic questions concerning building blocks, whose answers would further expand the supply and variety of examples of -manifolds.

### January 9, 2019

TITLE: The matching problem for twisted connected sums

ABSTRACT: Applying the twisted connected sum construction of -manifolds requires finding special diffeomorphisms between anticanonical K3 surfaces in building blocks. The aim of this talk is to explain how this problem can be solved by establishing sufficient conditions for a generic element of a family of K3s to appear as an anticanonical divisor in some element of a set of blocks. We will also discuss the matching problems involved in constructing associatives in or instantons on twisted connected sums by gluing, in joint work with Menet and Sa Earp.

### January 12, 2018

TITLE: G_{2}-instantons on twisted connected sums

ABSTRACT: The twisted connected sum construction of -manifolds glues two pieces along a common boundary that is a product of a K3 surfaces and a 2-torus. I will discuss joint work with Gregoire Menet and Henrique Sa Earp on constructing -instantons on such -manifolds. In the example I will describe, both halves of in the construction admit positive-dimensional moduli spaces of Hermitian-Yang-Mills instantons. Ensuring that their images in the moduli space of ASD instantons on the boundary K3 intersect transversely allows the application of a gluing theorem for -instantons due to Sa Earp and Walpuski.

### September 11, 2017

TITLE: New asymptotically conical G_{2}-manifolds

ABSTRACT: Until now, the only examples of asymptotically conical -manifolds that have been proved to exist are the three cohomogenity one examples of Bryant and Salamon. I will discuss cohomogeneity one examples on the total spaces of complex line bundles over S^{2} x S^{3}, whose asymptotic links are quotients of the standard nearly Kähler S^{3} x S^{3}.

### June 5, 2017

TITLE: Twisted connected sum G_{2}-manifolds

### June 6, 2017

TITLE: Topological invariants of G_{2}-manifolds

### September 6, 2016

TITLE: Complete and conically singular G_{2}-manifolds of cohomogeneity one

ABSTRACT: Bryant and Salmon’s cohomogeneity 1 examples of complete, asymptotically conical -manifolds provide a model for desingularising compact -manifolds with conical singularities; however no examples of the latter are yet known, and there are also no further known examples of asymptotically conical -manifolds. Theoretical physicists such as Cvetic-Gibbons-Lu-Pope and Brandhuber-Gomis-Gubser-Gukov have considered complete cohomogeneity 1 -manifolds that are “asymptotically locally conical”–the model at infinity is a circle bundle over a cone–and which in a 1-parameter family converge to an asymptotically conical manifold. However, only some of these families have been studied rigorously (Bazaikin-Bogoyavlenskaya).

I will discuss joint work in progress with Foscolo and Haskins on these families, and some of their limits, which include a new asymptotically conical -manifold and a conically singular -manifold with locally conical asymptotics. The latter may provide an avenue to construction of compact -manifolds with conical singularities.