# Free incomplete Tambara functors are almost never flat

## (with Mehrle and Quigley)

arXiv
Free algebras are always free as modules over the base ring in classical algebra. In equivariant algebra, free incomplete Tambara functors play the role of free algebras and Mackey functors play the role of modules. Surprisingly, free incomplete Tambara functors often fail to be free as Mackey functors. In this paper, we determine for all finite groups conditions under which a free incomplete Tambara functor is free as a Mackey functor. For solvable groups, we show that a free incomplete Tambara functor is flat as a Mackey functor precisely when these conditions hold. Our results imply that free incomplete Tambara functors are almost never flat as Mackey functors. However, we show that after suitable localizations, free incomplete Tambara functors are always free as Mackey functors.

# Freeness and equivariant stable homotopy

arXiv
We introduce a notion of freeness for $RO$-graded equivariant generalized homology theories, considering spaces or spectra $E$ such that the $R$-homology of $E$ splits as a wedge of the $R$-homology of induced virtual representation spheres. The full subcategory of these spectra is closed under all of the basic equivariant operations, and this greatly simplifies computation. Many examples of spectra and homology theories are included along the way.

We refine this to a collection of spectra analogous to the pure and isotropic spectra considered by Hill--Hopkins--Ravenel. For these spectra, the $RO$-graded Bredon homology is extremely easy to compute, and if these spaces have additional structure, then this can also be easily determined. In particular, the homology of a space with this property naturally has the structure of a co-Tambara functor (and compatibly with any additional product structure). We work this out in the example of $BU_{\mathbb R}$ and coinduced versions of this.

We finish by describing a readily computable bar and twisted bar spectra sequence, giving Bredon homology for various $E_{\infty}$ pushouts, and we apply this to describe the homology of $BBU_{\mathbb R}$.

# On the algebras over equivariant little disks

arXiv
We describe the structure present in algebras over the little disks operads for various representations of a finite group $G$, including those that are not necessarily universe or that do not contain trivial summands. We then spell out in more detail what happens for $G=C_2$, describing the structure on algebras over the little disks operad for the sign representation. Here we can also describe the resulting structure in Bredon homology. Finally, we produce a stable splitting of coinduced spaces analogous to the stable splitting of the product, and we use this to determine the homology of the signed James construction.

# Transchromatic extensions in motivic and Real bordism

## (With Beaudry, Shi, and Zeng)

arXiv
We show a number of Toda brackets in the homotopy of the motivic bordism spectrum $MGL$ and of the Real bordism spectrum $MU_{\mathbb R}$. These brackets are ``red-shifting'' in the sense that while the terms in the bracket will be of some chromatic height $n$, the bracket itself will be of chromatic height $(n+1)$. Using these, we deduce a family of exotic multiplications in the $\pi_{(\ast,\ast)}MGL$-module structure of the motivic Morava $K$-theories, including non-trivial multiplications by $2$. These in turn imply the analogous family of exotic multiplications in the $\pi_{\star}MU_\mathbb R$-module structure on the Real Morava $K$-theories.

# Models of Lubin-Tate spectra via Real bordism theory

## (With Beaudry, Shi, and Zeng)

arXiv
We construct $C_{2^n}$-equivariant Real oriented models of Lubin--Tate spectra $E_h$ at heights $h=2^{n-1}m$. We give explicit formulas of the $C_{2^n}$-action on their coefficient rings.
Our construction utilizes equivariant formal group laws associated with the norms of the Real bordism theory $MU_{\mathbb{R}}$.

# Bi-incomplete Tambara functors

## (With Blumberg)

arXiv
For an equivariant commutative ring spectrum $R$, $\pi_0 R$ has algebraic
structure reflecting the presence of both additive transfers and multiplicative
norms. The additive structure gives rise to a Mackey functor and the
multiplicative structure yields the additional structure of a Tambara functor.
If $R$ is an $N_\infty$ ring spectrum in the category of genuine $G$-spectra,
then all possible additive transfers are present and $\pi_0 R$ has the
structure of an incomplete Tambara functor. However, if $R$ is an $N_\infty$
ring spectrum in a category of incomplete $G$-spectra, the situation is more
subtle.

In this paper, we study the algebraic theory of Tambara structures on
incomplete Mackey functors, which we call bi-incomplete Tambara functors. Just
as incomplete Tambara functors have compatibility conditions that control which
systems of norms are possible, bi-incomplete Tambara functors have algebraic
constraints arising from the possible interactions of transfers and norms. We
give a complete description of the possible interactions between the additive
and multiplicative structures.

# Quotient rings of $H\mathbb F_2\wedge H\mathbb F_2$

## (With Beaudry, Lawson, Shi, and Zeng)

arXiv
We study modules over the commutative ring spectrum $H\mathbb F_2\wedge H\mathbb F_2$, whose coefficient groups are quotients of the dual Steenrod algebra by collections of the Milnor generators. We show that very few of these quotients admit algebra structures, but those that do can be constructed simply: killing a generator $\xi_k$ in the category of associative algebras freely kills the higher generators $\xi_{k+n}$. Using new information about the conjugation operation in the dual Steenrod algebra, we also consider quotients by families of Milnor generators and their conjugates. This allows us to produce a family of associative $H\mathbb F_2\wedge H\mathbb F_2$-algebras whose coefficient rings are finite-dimensional and exhibit unexpected duality features. We then use these algebras to give detailed computations of the homotopy groups of several modules over this ring spectrum.

# Equivariant stable categories for incomplete systems of transfers

## (With Blumberg)

arXiv
In this paper, we construct incomplete versions of the equivariant stable
category; i.e., equivariant stabilization of the category of $G$-spaces with
respect to incomplete systems of transfers encoded by an $N_\infty$ operad
$\mathcal{O}$. These categories are built from the categories of
$\mathcal{O}$-algebras in $G$-spaces. Using this operadic formulation, we
establish incomplete versions of the usual structural properties of the
equivariant stable category, notably the tom Dieck splitting. Our work is
motivated in part by the examples arising from the equivariant units and Picard
space functors.

# The slice spectral sequence of a $C_4$-equivariant height-4 Lubin-Tate theory

## (With Shi, Wang, and Xu)

arXiv
We completely compute the slice spectral sequence of the $C_4$-spectrum $BP^{((C4))}\langle 2\rangle$. After periodization and $K(4)$-localization, this spectrum is equivalent to a height-4 Lubin-Tate theory $E_4$ with $C_4$-action induced from the Goerss-Hopkins-Miller theorem. In particular, our computation shows that $E^{hC_{12}}_4$ is 384-periodic.

# Invertible $K(2)$-Local $E$-Modules in $C_4$-Spectra

## (Joint with Beaudry, Bobkova, and Stojanoska)

arXiv
We compute the Picard group of the category of $K(2)$-local module spectra over the ring spectrum $E^{hC_4}$, where $E$ is a height $2$ Morava $E$-theory and $C_4$ is a subgroup of the associated Morava stabilizer group. This group can be identified with the Picard group of $K(2)$-local $E$-modules in genuine $C_4$-spectra. We show that in addition to a cyclic subgroup of order $32$ generated by $E\wedge S^1$ the Picard group contains a subgroup of order 2 generated by $E\wedge S^{7+\sigma}$, where $\sigma$ is the sign representation of the group $C_4$. In the process, we completely compute the $RO(C_4)$-graded Mackey functor homotopy fixed point spectral sequence for the $C_4$-spectrum $E$.

# Detecting exotic spheres in low dimensions using coker J

## (Joint with Behrens, Hopkins, and Mahowald)

arXiv
Building off of the work of Kervaire and Milnor, and Hill, Hopkins, and Ravenel, Xu and Wang showed that the only odd dimensions n for which $S^n$ has a unique differentiable structure are 1, 3, 5, and 61. We show that the only even dimensions below 140 for which $S^n$ has a unique differentiable structure are 2, 6, 12, 56, and perhaps 4.

# The $\mathbb Z$-homotopy fixed points of $C_{n}$ spectra with applications to norms of $MU_{\mathbb R}$

## (Joint with Zeng)

arXiv
We introduce a computationally tractable way to describe the $\mathbb Z$-homotopy fixed points of a $C_{n}$-spectrum $E$, producing a genuine $C_{n}$ spectrum $E^{hn\mathbb Z}$ whose fixed and homotopy fixed points agree and are the $\mathbb Z$-homotopy fixed points of $E$. These form a piece of a contravariant functor from the divisor poset of $n$ to genuine $C_{n}$-spectra, and when $E$ is an $N_{\infty}$-ring spectrum, this functor lifts to a functor of $N_{\infty}$-ring spectra.

For spectra like the Real Johnson--Wilson theories or the norms of Real bordism, the slice spectral sequence provides a way to easily compute the $RO(G)$-graded homotopy groups of the spectrum $E^{hn\mathbb Z}$, giving the homotopy groups of the $\mathbb Z$-homotopy fixed points. For the more general spectra in the contravariant functor, the slice spectral sequences interpolate between the one for the norm of Real bordism and the especially simple $\mathbb Z$-homotopy fixed point case, giving us a family of new tools to simplify slice computations.

# The cohomology of $C_2$-equivariant $\mathcal A(1)$ and the homotopy of $ko_{C_2}$

## (Joint with Guillou, Isaksen, and Ravenel)

arXiv
We compute the cohomology of the subalgebra $\mathcal A^{C_2}(1)$ of the $C_2$-equivariant Steenrod algebra $\mathcal A^{C_2}$. This serves as the input to the $C_2$-equivariant Adams spectral sequence converging to the $RO(C_2)$-graded homotopy groups of an equivariant spectrum $ko_{C_2}$. Our approach is to use simpler $\mathbb C$-motivic and $\mathbb R$-motivic calculations as stepping stones.

# The Witt vectors for Green functors

## (Joint with Blumberg, Gerhardt, and Lawson)

arXiv
We define twisted Hochschild homology for Green functors. This construction is the algebraic analogue of the relative topological Hochschild homology $THH_{C_n}(−)$, and it describes the $E_2$ term of the K\"unneth spectral sequence for relative $THH$. Applied to ordinary rings, we obtain new algebraic invariants. Extending Hesselholt's construction of the Witt vectors of noncommutative rings, we interpret our construction as providing Witt vectors for Green functors.

# G-symmetric monoidal categories of modules over equivariant commutative ring spectra

## (Joint with Blumberg)

arXiv
We describe the multiplicative structures that arise on categories of equivariant modules over certain equivariant commutative ring spectra. Building on our previous work on N-infinity ring spectra, we construct categories of equivariant operadic modules over N-infinity rings that are structured by equivariant linear isometries operads. These categories of modules are endowed with equivariant symmetric monoidal structures, which amounts to the structure of an "incomplete Mackey functor in homotopical categories". In particular, we construct internal norms which satisfy the double coset formula. We regard the work of this paper as a first step towards equivariant derived algebraic geometry.

# Topological cyclic homology via the norm

## (Joint with Angeltveit, Blumberg, Gerhardt, Lawson, and Mandell)

arXiv
We describe a construction of the cyclotomic structure on topological Hochschild homology ($THH$) of a ring spectrum using the Hill-Hopkins-Ravenel multiplicative norm. Our analysis takes place entirely in the category of equivariant orthogonal spectra, avoiding use of the Bokstedt coherence machinery. As a consequence, we are able to define versions of topological cyclic homology ($TC$) relative to an arbitrary commutative ring spectrum $A$. We describe spectral sequences computing this relative theory ${}_{A}TR$ in terms of $TR$ over the sphere spectrum and vice versa. Furthermore, our construction permits a straightforward definition of the Adams operations on $TR$ and $TC$.

# Equivariant chromatic localizations and commutativity

arXiv
In this paper, we study the extent to which Bousfield and finite localizations relative to a thick subcategory of equivariant finite spectra preserve various kinds of highly structured multiplications. Along the way, we describe some basic, useful results for analyzing categories of acyclics in equivariant spectra, and we show that Bousfield localization with respect to an ordinary spectrum (viewed as an equivariant spectrum with trivial action) always preserves equivariant commutative ring spectra.

# The right adjoint to the equivariant operadic forgetful functor on incomplete Tambara functors

## (Joint with Blumberg)

For $N_\infty$ operads $\mathcal O$ and $\mathcal O'$ such that there is an inclusion of the associated indexing systems, there is a forgetful functor from incomplete Tambara functors over $\mathcal O'$ to incomplete Tambara functors over $\mathcal O$. Roughly speaking, this functor forgets the norms in $\mathcal O'$ that are not present in $\mathcal O$. The forgetful functor has both a left and a right adjoint; the left adjoint is an operadic tensor product, but the right adjoint is more mysterious. We explicitly compute the right adjoint for finite cyclic groups of prime order.

# Incomplete Tambara Functors

## (Joint with Blumberg)

arXiv
In this paper, we describe the algebraic analogue of $N_\infty$ ring structures. We introduce and study categories of incomplete Tambara functors, described in terms of certain categories of bispans. Incomplete Tambara functors arise as $\pi_0$ of $N_\infty$ algebras, and interpolate between Green functors and Tambara functors. We classify all incomplete Tambara functors in terms of a basic structural result about polynomial functors. This classification gives a conceptual justification for our prior description of $N_\infty$ operads and also allows us to easily describe the properties of the category of incomplete Tambara functors.

# A new formulation of the equivariant slice filtration with applications to $C_p$-slices

## (Joint with Yarnall)

arXiv
This paper provides a new way to understand the equivariant slice filtration. We give a new, readily checked condition for determining when a $G$-spectrum is slice $n$-connective. In particular, we show that a $G$-spectrum is slice greater than or equal to $n$ if and only if for all subgroups $H$, the $H$-geometric fixed points are $(n/|H|-1)$-connected. We use this to determine when smashing with a virtual representation sphere $S^V$ induces an equivalence between various slice categories. Using this, we give an explicit formula for the slices for an arbitrary $C_p$-spectrum and show how a very small number of functors determine all of the slices for $C_{p^n}$-spectra.

# On the Andre-Quillen homology of Tambara functors

arXiv
In this paper, we describe Mackey functor objects in the category of augmented Tambara functors, genuine equivariant derivations, and genuine Kahler differentials. We show that these are connected just as classically. This is the foundation for future work on equivariant TAQ

# The $C_2$-spectrum $TMF_1(3)$ and its invertible modules

## (Joint with Meier)

arXiv
We show that many of the interesting derived algebraic geometry properties (Anderson duality and the computation of the Picard group) for the $C_2$-spectrum $Tmf_1(3)$ follow from fairly straightforward equivariant considerations.

# The slice spectral sequence for certain $RO(C_{p^n})$-graded suspensions of $H\underline{\mathbb Z}$

## (Joint with Hopkins and Ravenel)

arXiv
We study the slice filtration and associated spectral sequence for a family of $RO(C_{p^{n}})$-graded suspensions of the Eilenberg-MacLane spectrum for the constant Mackey functor $\underline{\mathbb Z}$. Since $H\underline{\mathbb Z}$ is the zero slice of the sphere spectrum, this begins an analysis of how one can describe the slices of a suspension in terms of the original slices.

# Interpreting the Bokstedt smash product as the norm

## (Joint with Angeltveit, Blumberg, Gerhardt, and Lawson

arXiv
In this first of a series of three papers, we compare various notions of equivariant smash powers. This generalizes work of Shipley and shows that the norm provides both conceptual and homotopical control in a way similar to that of the Bokstedt model.

# The slice spectral sequence for the $C_4$ analogue of Real $K$-theory

## (Joint with Hopkins and Ravenel)

arXiv
We completely describe the slice spectral sequence computing the homotopy groups of the spectrum of topological modular forms with a $\Gamma_0(5)$-structure, $TMF_0(5)$. We describe the slice spectral sequence of a 32-periodic $C_4$-spectrum $K_H$ related to the $C_4$ norm $N_{C_{2}}^{C_4}MU_R$ of the real cobordism spectrum $MU_R$. We will give it as a spectral sequence of Mackey functors converging to the graded Mackey functor $\pi_\ast K_H$, complete with differentials and exotic extensions in the Mackey functor structure. The slice spectral sequence for the 8-periodic real $K$-theory spectrum $K_R$ was first analyzed by Dugger. The $C_8$ analog of $K_H$ is 256-periodic and detects the Kervaire invariant classes $\theta_j$. A partial analysis of its slice spectral sequence led to the solution to the Kervaire invariant problem, namely the theorem that $\theta_j$ does not exist for $j\geq 7$.

# On the non-existence of elements of Kervaire invariant one

## (Joint with Hopkins and Ravenel)

arXiv
In this paper, we prove that there are no smooth Kervaire invariant one manifolds of dimension larger than 126 using equivariant homotopy theory.

# Operadic multiplications in equivariant spectra, norms, and transfers

## (Joint with Blumberg)

arXiv
We study homotopy-coherent commutative multiplicative structures on equivariant spaces and spectra. These are generalizations of the usual $E_\infty$ operads, but have *a priori* less structure than classical $G-E_\infty$-operads. Algebras over our operads, called $N_\infty$ operads, have norms maps which, in the case of spectra, are the Hill-Hopkins-Ravenel norms, and in the case of spaces, are the transfer.

# Topological modular forms with level structure

## (Joint with Lawson)

arXiv
We show that the Goerss-Hopkins-Miller sheaf of $E_\infty$ ring spectra $\mathcal O^{top}$ on the moduli stack of elliptic curves extends over the log-etale site. This allows us to functorially produce models for so-called ``elliptic curves with level structure''.

# Topological Modular Forms

## (Jointly edited with Douglas, Francis, and Henriques)

PDF
This is the long-awaited proceedings for the 2007 Talbot conference on topological modular forms. The book contains chapters covering all of the background materials, together with Behrens' description of the construction of the $tmf$-sheaf, Hopkins and Miller's original paper on $tmf$, Hopkins' paper on $K(1)$-local $E_\infty$-ring spectra, and Hopkins and Mahowald's paper on elliptic curves and stable homotopy. The PDF linked here is done with permission from the AMS. If you would like to purchase a physical copy of the book, you may do so at the AMS Bookstore.

# Equivariant Multiplicative Closure

## (Joint with Hopkins)

arXiv
This paper describes an issue that arises when inverting elements of the homotopy groups of an equivariant commutative ring.

# On the Algebraic $K$-theory of Truncated Polynomial Algebras in Several Variables

## (Joint with Angeltveit, Gerhardt, and Lindenstrauss)

arXiv
We compute using cyclotomic trace methods the algebraic K-theory of truncated polynomial algebras. We also describe a convenient category for cyclotimic things and show how this simplifies several kinds of computations.

# The equivariant slice filtration: a primer

arXiv
In this paper, I give an overview of the equivariant slice filtration. I produce a large family of slices, and then I determine the slice tower associated to Eilenberg-MacLane spectra.

# The Kervaire invariant one problem in algebraic topology: proof

##
(Joint with Hopkins and Ravenel)

PDF
This is a detailed sketch of our arguments written for the Current Developments in Mathematics conference at Harvard. It contains several distinct arguments, and is largely self-contained.

# The Kervaire invariant one problem in algebraic topology: introduction

## (Joint with Hopkins and Ravenel)

PDF
This is a write-up of the historical talk given by Ravenel at the CDM conference at Harvard.

# Homological obstructions to string orientations

## (Joint with Douglas and Henriques)

arXiv
This paper analyzes Poincare duality in the context of modules over the Steenrod algebra

# Ext and the motivic Steenrod algebra over $\mathbb R$

arXiv
This demonstrates a motivic Bockstein spectral sequence used to compute motivic Ext over finite subalgebras of the Motivic Steenrod algebra.

# Automorphic forms and cohomology theories on Shimura curves of small discriminant

## (Joint with Lawson)

arXiv
We compute several rings of automorphic forms and apply this to compute the homotopy groups of several spectra produced by Lurie's Artin Representability.

# THH of $\ell$ and $ko$

## (Joint with Angeltveit and Lawson)

arXiv
This paper simultaneously runs two pairs of Bockstein spectral sequences to compute topological Hochschild homology.

# The spectra $ko$ and $ku$ are not Thom spectra

## (Joint with Angeltveit and Lawson)

arXiv
Using Hochschild homology, we show that ko and ku are not Thom spectra.

# The String Bordism of $BE_8$

arXiv
This computes the String Bordism groups of $BE_8$ and $BE_8\times BE_8$ through dimension 14. This computation amounts to computing the tmf-homology of $K(\mathbb Z,3)$.

# A $v_2^{32}$ self-map of $M(1,4)$ at the prime 2

## (Joint with Behrens, Hopkins, & Mahowald)

arXiv
This proves the existence of a $v_2^{32}$-self map on the generalized Smith-Toda complex $M(1,4)$.

# The 5-Local Homotopy of $eo_4$

arXiv
A Bockstein and Adams-Novikov spectral sequence computation for the homotopy ring of the conjectural spectrum $eo_4$.

# Cyclic comodules, the Homology of $j$ and $j$-Homology

PDF
An elementary presentation of the cohomology of the connective $j$-theory spectrum.

# The tmf-homology of $B\Sigma_3$

PDF
This gives a form of the Adams spectral sequence for tmf-homology and applies it to compute the tmf-homology of $B\Sigma_3$.