### Publications and preprints

*Cross-effects and the classification of Taylor towers*, (joint with Greg Arone), submitted, arxiv:1404.1417.We show that the partially-stabilized cross-effects of a functor from either based spaces of spectra, to spectra, admit the structure of a module over a certain operad: the Koszul dual of the little discs operad in the first case, and a desuspension of the commutative operad in the second case. The derivatives inherit a limiting structure as a module over a 'pro-operad'. We show that the Taylor tower of the functor can be recovered from this structure.

*Coalgebraic models for combinatorial model categories*, (joint with Emily Riehl), to appear in Homology, Homotopy and Applications, arxiv:1403.5303.We show that the category of algebraically cofibrant objects in a combinatorial and simplicial model category A has a model structure that is left-induced from that on A. In particular it follows that any presentable model category is Quillen equivalent (via a single Quillen equivalence) to one in which all objects are cofibrant.

*Higher homotopy excision and Blakers-Massey theorems for structured ring spectra*, (joint with John E. Harper), submitted, arXiv:1402.4775.We prove versions of Goodwillie's higher homotopy excision and higher Blakers-Massey theorems in the context of algebras over an operad in the category of spectra.

*A classification of Taylor towers for functors of spaces and spectra*, (joint with Greg Arone), submitted, arxiv:1209.5661.We describe new structure on the Goodwillie derivatives of a functor, and we show how the full Taylor tower of the functor can be recovered from this structure. This new structure takes the form of a coalgebra over a certain comonad which we construct, and whose precise nature depends on the source and target categories of the functor in question. The Taylor tower can be recovered from standard cosimplicial cobar constructions on the coalgebra formed by the derivatives. We get from this an equivalence between the homotopy category of polynomial functors and that of bounded coalgebras over this comonad. For functors with values in the category of spectra, we give a rather explicit description of the associated comonads and their coalgebras. In particular, for functors from based spaces to spectra we interpret this new structure as that of a divided power right module over the operad formed by the derivatives of the identity on based spaces.

*Apollonian circle packings of the half-plane*, (joint with John Doyle), Journal of Combinatorics**3**(2012), 1-49.We classify Apollonian circle packings of a half-plane up to Euclidean similarity and describe the relationship between such packings and continued fractions.

*A note on the composition product of symmetric sequences*, Journal of Homotopy and Related Structure**7**(2012), 237-254.We consider the composition product of symmetric sequences in the case where the underlying symmetric monoidal structure does not commute with coproducts. Even though this composition product is not a monoidal structure on symmetric sequences, it has enough structure, namely that of a `normal oplax' monoidal product, to be able to define monoids (which are then operads on the underlying category) and make a bar construction. The main benefit of this work is in the dual setting, where it allows us to define a cobar construction for cooperads.

*Bar-cobar duality for operads in stable homotopy theory*, Journal of Topology**5**(2012), 39-80.We describe a bar-cobar duality for operads of spectra. In particular, we construct a Quillen equivalence between the projective model structure on the category of operads, and a new model for the homotopy theory of cooperads of spectra.

*Operads and chain rules for calculus of functors*, (joint with Greg Arone), Astérisque**338**(2011), 158 pages.We study the structure possessed by the Goodwillie derivatives of a pointed homotopy functor of based topological spaces. These derivatives naturally form a bimodule over the operad consisting of the derivatives of the identity functor. We then use these bimodule structures to give a chain rule for higher derivatives in the calculus of functors, extending that of Klein and Rognes. This chain rule expresses the derivatives of FG as a derived composition product of the derivatives of F and G over the derivatives of the identity. There are two main ingredients in our proofs. Firstly, we construct new models for the Goodwillie derivatives of functors of spectra. These models allow for natural composition maps that yield operad and module structures. Then, we use a cosimplicial cobar construction to transfer this structure to functors of topological spaces. A form of Koszul duality for operads of spectra plays a key role in this.

*A chain rule for Goodwillie derivatives of functors from spectra to spectra*, Trans. Amer. Math. Soc.**362**(2010), 399-426.We prove a chain rule for the Goodwillie calculus of functors from spectra to spectra. We show that the (higher) derivatives of a composite functor FG at a base object X are given by taking the composition product (in the sense of symmetric sequences) of the derivatives of F at G(X) with the derivatives of G at X. We also consider the question of finding P

_{n}(FG), and give an explicit formula for this when F is homogeneous.*Bar constructions for topological operads and the Goodwillie derivatives of the identity*, Geom. Topol.**9**(2005), 833-933.We describe a cooperad structure on the simplicial bar construction on a reduced operad of based spaces or spectra and, dually, an operad structure on the cobar construction on a cooperad. We also show that if the homology of the original operad (respectively, cooperad) is Koszul, then the homology of the bar (respectively, cobar) construction is the Koszul dual. We use our results to construct an operad structure on the partition poset models for the Goodwillie derivatives of the identity functor on based spaces and show that this induces the ‘Lie’ operad structure on the homology groups of these derivatives. We also extend the bar construction to modules over operads (and, dually, to comodules over cooperads) and show that a based space naturally gives rise to a left module over the operad formed by the derivatives of the identity.

### Current Projects

*Derivatives of functors represented by manifolds*(joint with Greg Arone)The goal of this project is to understand the structure on derivatives of functors represented by certain 'pointed framed manifolds'. We show that these have the structure of a module over the Koszul dual of the little n-discs operad where n is the dimension of the manifold.

*Localized Taylor towers*(joint with Greg Arone)This project is to extend Kuhn's results on the splitting of Taylor towers of functors of K(n)-local spectra to the unstable case. We do not expect split Taylor towers in this case, but we do expect the Taylor towers to be determined by the Goodwillie derivatives together with the appropriate bimodule structure. The idea is to apply Kuhn's theorem on vanishing Tate cohomology to the best approximation to a functor determined by the derivatives.

*Completion of operadic algebras with respect to Quillen homology*(joint with John E. Harper)The goal of this project is to give connectivity conditions under which an algebra A over an operad P (in spectra or chain complexes) can be recovered from its Quillen homology. This is part of the foundation of a Koszul duality for relating algebras over P to coalgebras over the bar construction BP.

*The Taylor tower and Goodwillie derivatives of algebraic K-theory*(joint with Andrew Blumberg)This is an attempt to calculate some parts of the Taylor tower and Goodwillie derivatives of algebraic K-theory viewed as a functor from associative (or commutative) S-algebras to spectra. We use the framework developed in my work with Greg Arone and use results of Basterra-Mandell on the stabilization of categories of S-algebras, and of Lindenstrauss-McCarthy on Taylor towers of other algebraic K-theory functors.

*Geometric self-duality for the little discs operads*(joint with Benoit Fresse and Paolo Salvatore)Getzler and Jones showed that the homology of the little n-discs operad is self-Koszul dual, up to suspension. This project is an attempt to realize that duality geometrically using my work on bar-cobar duality for operads of spectra.

### Manuscripts

- Calculus of functors and configuration spaces
This is a summary of a talk given at the Conference on Pure and Applied Topology on the Isle of Skye in June 2005. We describe a relationship between right modules over the operad formed by the Goodwillie derivatives of the identity functor, and configuration spaces on manifolds. The missing step in turning this project into a paper is a proper understanding of the self-Koszul-duality of the Fulton-MacPherson operads of compactified configuration spaces.

- Cross-effects of homotopy functors and spaces of trees
This paper describes models for the cross-effects of homotopy functors given in terms of spaces of trees. These link closely to the methods of the paper "Bar constructions for topological operads and the Goodwillie derivatives of the identity", but I do not have any useful applications for these models.

- Homotopy operations on simplicial algebras over an operad
This was my first attempt as a graduate student to understand what operations exist on the homotopy groups of a simplicial algebra over an operad, in a sense generalizing the Cartan-Bousfield-Dwyer operations on simplicial commutative algebras. It contains some partial results, but not a full description.

## Michael Ching

Department of Mathematics

Amherst College

PO Box 5000

Amherst, MA 01002

USA

Telephone: (413) 542-5530

Fax: (413) 542-2550

Email: mching@amherst.edu