**Research**

I am an algebraic topologist. I refrain from trying to explain in
detail what algebraic topology is. For some points of view on that the
*Essays about
algebraic topology* might be useful.
My current reseach interest is stable
homotopy theory, more specifically: **structured ring spectra**.

Since the mid 90s there are several strictly symmetric monoidal categories of spectra available which yield a model of the stable homotopy category. I am interested in multiplicative cohomology theories and particularly such cohomology theories that stem from so called commutative S-algebras; these are nothing but commutative monoids in the category of spectra.

Having *Brave New Rings* (aka structured ring spectra) one wants
to understand their arithmetic properties. In joint work with Andy Baker
we investigated Galois extensions of ring spectra in the sense of John
Rognes, and Picard groups. We also try to understand Azumaya
extensions of ring spectra and the corresponding notion of Brauer
groups. This is related to work of Bertrand Toen, David Gepner,
Niles Johnson and others.

I got involved in algebraic K-theory of ring spectra. There is a machinery that turns bimonoidal categories into ring spectra. If you plug in a ring, you get the corresponding Eilenberg-MacLane spectrum. A conjecture of Baas-Dundas-Rognes states that algebraic K-theory of such spectra can be described in one go via a suitable K-theory construction in a 2-categorical setting. In joint work with Baas, Dundas and Rognes we prove this conjecture. In particular this identifies K(ku) (the algebraic K-theory of connective complex topological K-theory) with the K-theory of 2-vector spaces. Ausoni and Rognes proved that K(ku) can be viewed as a version of elliptic cohomology.

In algebra it is most of the time obvious whether a ring is commutative or not. In topology, identifying a ring spectrum as a commutative one can be quite tricky. For instance, residue fields of commutative ring spectra are rarely commutative. Sometimes one has to use the old-fashioned machinery of so-called \(E_\infty\)(= homotopy everything) ring spectra to establish such structures. Being \(E_\infty\) is a property which is formulated in the language of operads. As a technical means these can be quite useful and part of my research is concerned with them. In a project with Muriel Livernet we study homology theories for so-called \(E_n\)-algebras (these are algebraic analogues of n-fold loop spaces). We provide an interpretation of such homology theories in terms of functor homology. we build some machinery for actual calculations of these homology groups. In a joint project with Brooke Shipley we show that there is a Quillen equivalence between differential graded \(E_\infty\)-algebras and commutative \(H\mathbb{Z}\)-algebra spectra.

Related work is on higher order (topological) Hochschild homology. Our first WIT (women in topology) team, Irina, Inna, Kate, Ayelet and I, did some calculations of higher order THH of \(F_p\)-algebras, our second one, Gemma, Eva, Ayelet, Inna and I, produced some results that determine higher order THH in other cases. In work with Ayelet Lindenstrauss and Bjørn Dundas we show that higher THH of number rings detects ramification. Together with Eva Höning I study ramification phenomena in more detail. Ayelet and I got interested in the stability question: When does the homology of a commutative ring spectrum with respect to a simplicial set \(X\) only depend on the suspension of \(X\)? We have some structural results and can identify some classes of examples and some counterexamples.

Equivariant homotopy theory is a very active area of research since the famous solution by Hill, Hopkins, Ravenel of the Kervaire invariant problem. With Ayelet Lindenstrauss and Foling Zou I'm studying examples of étale extensions of Tambara functors and equivariant versions of Loday constructions. The twisted cyclic nerve of Blumberg-Gerhardt-Hill-Lawson and Real topological Hochschild homology turn out to be examples of this construction.

For finished projects and work in progress see my list of publications.