arXiv subjects

Andrew Salch

Publications and source records attributed to Andrew Salch.

At least 19 recordsLinked to original sources

arXiv2025

Notes on model structures on preorders

Given subsets $\mathcal{C},\mathcal{F}$ of a preorder $\mathcal{A}$, we give necessary and sufficient conditions for $\mathcal{A}$ to admit the structure of a model category whose cofibrant objects are $\mathcal{C}$ and whose fibrant objects are $\mathcal{F}$. We give various classification results for model structures on preorders by describing model structures in terms of their fibrant and cofibrant objects, or in terms of their (co)fibrant replacment (co)monads. This leads to a construction which takes topologies and matroids as input, and produces model structures on Boolean algebras. We carry out some detailed case studies, calculating all model structures on small Boolean algebras, and all the Bousfield localization and colocalization relations between them.

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arXiv2025

The action of the Morava stabilizer group on the coefficients of Morava E-theory at height 2

We calculate an explicit closed formula for the action of the height 2 full Morava stabilizer group on the coefficient ring of height 2 Morava E-theory. In particular, this yields an explicit, surprisingly simple closed formula for the action of the automorphism group of a height 2 formal group law on its Lubin-Tate deformation ring. The formula is of a combinatorial nature, given by sums over certain labelled ordered rooted trees.

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arXiv2024

The mod p cohomology of the Morava stabilizer group at large primes

We calculate the cohomology of the extended Morava stabilizer group of height $n$, with trivial mod $p$ coefficients, for all heights $n$ and all primes $p>>n$. The result is an exterior algebra on $n$ generators. A brief sketch of the method: we introduce a family of deformations of Ravenel's Lie algebra model $L(n,n)$ for the Morava stabilizer group scheme. This yields a family of DGAs, parameterized over an affine line and smooth except at a single point. The singular fiber is the Chevalley-Eilenberg DGA of Ravenel's Lie algebra. Consequently the cohomology of the singular fiber is the cohomology of the Morava stabilizer group, at large primes. We prove a derived version of the invariant cycles theorem from Hodge theory, which allows us to compare the cohomology of the singular fiber to the fixed-points of the Picard-Lefschetz (monodromy) operator on the cohomology of a smooth fiber. Finally, we use some new methods for constructing small models for cohomology of reductive Lie algebras to show that the cohomology of the Picard-Lefschetz fixed-points on a smooth fiber agrees with the singular cohomology $H^*(U(n);\mathbb{F}_p)$ of the unitary group, which is the desired exterior algebra.

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arXiv2024

Iwasawa invariants of finite spectra

We calculate the classical Iwasawa invariants of the Iwasawa modules associated to the $p$-adic topological $K$-theory of finite spectra. We show that the graded average of the orders of $n$ consecutive $K(1)$-local homotopy groups of a finite spectrum $X$ grows asymptotically like $\frac{-\log_p(n)}{2}$ times the total Iwasawa $λ$-invariant of $X$. We show that the Iwasawa $μ$-invariants of finite spectra are all zero. Finally, we prove a spectral analogue of a weak form of the Iwasawa Main Conjecture, describing the orders of the $K(1)$-local homotopy groups of a certain ``torsion-free replacement'' of $X$ in terms of the characteristic polynomials of the Iwasawa modules associated to $X$.

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arXiv2024

Products in spin$^c$-cobordism

We calculate the mod $2$ spin$^c$-cobordism ring up to uniform $F$-isomorphism (i.e., inseparable isogeny). As a consequence we get the prime ideal spectrum of the mod $2$ spin$^c$-cobordism ring. We also calculate the mod $2$ spin$^c$-cobordism ring ``on the nose'' in degrees $\leq 33$. We construct an infinitely generated nonunital subring of the $2$-torsion in the spin$^c$-cobordism ring. We use our calculations of product structure in the spin and spin$^c$ cobordism rings to give an explicit example, up to cobordism, of a compact $24$-dimensional spin manifold which is not cobordant to a sum of squares, which was asked about in a 1965 question of Milnor.

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arXiv2024

The Hochschild homology and cohomology of A(1)

We compute the Hochschild homology and cohomology of $A(1)$, the subalgebra of the $2$-primary Steenrod algebra generated by the first two Steenrod squares, $Sq^1$ and $Sq^2$. The computation is accomplished using several May-type spectral sequences.

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arXiv2023

ell-adic topological Jacquet-Langlands duality

We embed the Lubin-Tate tower into a larger tower of formal schemes, the "degenerating Lubin-Tate tower." We construct a topological realization of the degenerating Lubin-Tate tower, i.e., a compatible family of presheaves of $E_{\infty}$-ring spectra on the étale site of each formal scheme in the degenerating Lubin-Tate tower, which agrees on the base of the tower with the Goerss-Hopkins presheaf on Lubin-Tate space. We define and prove basic properties of nearby cycle and vanishing cycle presheaves of spectra on formal schemes. We apply these constructions to our spectrally-enriched degenerating Lubin-Tate tower to produce, for every spectrum $X$, an "$\ell$-adic topological Jacquet-Langlands (TJL) dual" of $X$. We prove that there is a correspondence between: 1. certain irreducible representations of $Aut(\mathbb{G})$ occurring in $(\mathcal{K}(E(\mathbb{G}))^{\wedge}_{\ell})_*(X)$, and 2. certain supercuspidal irreducible representations of $GL_n$ occuring in the rational homotopy groups of the TJL dual of $X$. Here $E(\mathbb{G})$ is the Morava $E$-theory spectrum of a height $n$ formal group $\mathbb{G}$, and $\mathcal{K}(E(\mathbb{G}))^{\wedge}_{\ell}$ is its algebraic $K$-theory spectrum completed away from the characteristic of the ground field of $\mathbb{G}$. Finally, we prove that at height $1$, TJL duality preserves the $L$-factors. This means that the automorphic $L$-factor of the $GL_1$-representation associated to $(\mathcal{K}(E(\mathbb{G}))^{\wedge}_{\ell})_*(X)$ by TJL duality is precisely the $p$-local Euler factor in a meromorphic $L$-function whose special values in the left half-plane recover the orders of the $KU$-local stable homotopy groups of $X$.

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arXiv2023

Approximation of subcategories by abelian subcategories

For a commutative ring $R$ and a weakly proregular ideal $I$, we prove a simple universal property of the category of $L_0$-complete $R$-modules: it is the smallest replete exact abelian subcategory of the category of $R$-modules which contains all the $I$-adically complete $R$-modules.

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arXiv2023

Structure and cohomology of moduli of formal modules

Given a commutative ring $A$, a "formal $A$-module" is a formal group equipped with an action of $A$. There exists a classifying ring $L^A$ of formal $A$-modules. This paper proves structural results about $L^A$ and about the moduli stack $\mathcal{M}_{fmA}$ of formal $A$-modules. We use these structural results to aid in explicit calculations of flat cohomology groups of $\mathcal{M}_{fmA}^{2-buds}$, the moduli stack of formal $A$-module $2$-buds. For example, we find that a generator of the group $H^1_{fl}(\mathcal{M}_{fm\mathbb{Z}}; ω)$, which also generates (via the Adams-Novikov spectral sequence) the first stable homotopy group of spheres, also yields a generator of the $A$-module $H^1_{fl}(\mathcal{M}_{fmA}^{2-buds}; ω)$ for any torsion-free Noetherian commutative ring $A$. We show that the order of the $A$-modules $H^1_{fl}(\mathcal{M}_{fmA}^{2-buds}; ω)$ and $H^2_{fl}(\mathcal{M}_{fmA}^{2-buds}; ω\otimes ω)$ are each equal to $2^{N_1}$, where $N_1$ is the leading coefficient in the $2$-local zeta-function of $Spec A$. We also find that the cohomology of $\mathcal{M}_{fmA}^{2-buds}$ is closely connected to the delta-invariant and syzygetic ideals studied in commutative algebra: $H^0_{fl}(\mathcal{M}_{fmA}^{2-buds}; ω\otimes ω)$ is the delta-invariant of the largest ideal of $A$ which is in the kernel of every ring homomorphism $A\rightarrow \mathbb{F}_2$, and consequently $H^0_{fl}(\mathcal{M}_{fmA}^{2-buds}; ω\otimes ω)$ vanishes if and only if $A$ is a ring in which that ideal is syzygetic.

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arXiv2023

Milnor-Moore theorems for bialgebras in characteristic zero

Over fields of characteristic zero, we construct equivalences between certain categories of bialgebras which are generated by grouplikes and generalized primitives, and certain categories of structured Lie algebras. The relevant families of bialgebras include many which are not connected, and which fail to admit antipodes.

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arXiv2022

The topological Petersson product

The nondegeneracy of the Petersson inner product on cusp forms, and the fact that Hecke operators are self-adjoint with respect to the Petersson product, together imply that the cusp forms have a basis consisting of Hecke eigenforms. In the literature on topological modular forms, no topological analogue of the Petersson product is to be found, and it is not known which topological spaces have the property that their topological cusp forms admit a basis consisting of eigenforms for the action of Baker's topological Hecke operators. In this note we define and study a natural topological Petersson product on complexified topological cusp forms, whose value on a one-point space recovers the classical Petersson product. We find that the topological Petersson product is usually degenerate: in particular, if $X$ is a space with nontrivial rational homology in any positive degree, then the Petersson product on complexified $tcf^*(X)$ is degenerate. Despite $tcf$-cohomology being a stable invariant, the topological Petersson product is an unstable invariant, vanishing on all suspensions. Nevertheless, we demonstrate nontriviality of the topological Petersson product by giving an explicit calculation of the topological Petersson product on complexified $tcf$-cohomology of the complex projective plane. We show that, for a compact Kahler manifold $X$, the Petersson product is nondegenerate on complexified $tcf$-cohomology of $X$ in a range of Atiyah-Hirzebruch filtrations (essentially one-third of the possibly-nonzero Atiyah-Hirzebruch filtrations in the $tcf$-cohomology of $X$).

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arXiv2022

Topological Hecke eigenforms

We study the eigenforms of the action of A. Baker's Hecke operators on the holomorphic elliptic homology of various topological spaces. We prove a multiplicity one theorem (i.e., one-dimensionality of the space of these "topological Hecke eigenforms" for any given eigencharacter) for some classes of topological spaces, and we give examples of finite CW-complexes for which multiplicity one fails. We also develop some abstract "derived eigentheory" whose motivating examples arise from the failure of classical Hecke operators to commute with multiplication by various Eisenstein series. Part of this "derived eigentheory" is an identification of certain derived Hecke eigenforms as the obstructions to extending topological Hecke eigenforms from the top cell of a CW-complex to the rest of the CW-complex. Using these obstruction classes together with our multiplicity one theorem, we calculate the topological Hecke eigenforms explicitly, in terms of pairs of classical modular forms, on all 2-cell CW complexes obtained by coning off an element in $π_n(S^m)$ which stably has Adams-Novikov filtration 1.

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arXiv2020

Commuting unbounded homotopy limits with Morava K-theory

This paper provides conditions for Morava $K$-theory to commute with certain homotopy limits. These conditions extend previous work on this question by allowing for homotopy limits of sequences of spectra that are not uniformly bounded below. As an application, we prove the $K(n)$-local triviality (for sufficiently large $n$) of the algebraic $K$-theory of algebras over truncated Brown--Peterson spectra, building on work of Bruner--Rognes and extending a classical theorem of Mitchell on $K(n)$-local triviality of the algebraic K-theory spectrum of the integers for large enough $n$.

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arXiv2018

A May-type spectral sequence for higher topological Hochschild homology

Given a filtration of a commutative monoid $A$ in a symmetric monoidal stable model category $\mathcal{C}$, we construct a spectral sequence analogous to the May spectral sequence whose input is the higher order topological Hochschild homology of the associated graded commutative monoid of $A$, and whose output is the higher order topological Hochschild homology of $A$. We then construct examples of such filtrations and derive some consequences: for example, given a connective commutative graded ring $R$, we get an upper bound on the size of the $THH$-groups of $E_{\infty}$-ring spectra $A$ such that $π_*(A) \cong R$.

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arXiv2018

Maps of simplicial spectra whose realizations are cofibrations

Given a map of simplicial topological spaces, mild conditions on degeneracies and the levelwise maps imply that the geometric realization of the simplicial map is a cofibration. These conditions are not formal consequences of model category theory, but depend on properties of spaces, and similar results have not been available for any model for the stable homotopy category. In this paper we prove such results for symmetric spectra. Consequently, we get a set of conditions which ensure that the geometric realization of a map of simplicial symmetric spectra is a cofibration. These conditions are very user-friendly in that they are simple, often easily checked, and do not require computation of a latching object or any other knowledge of Reedy theory.

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arXiv2017

CW-complexes in the Category of Small Categories

We compute the collection of CW-complexes in the model category of small categories constructed by Joyal and Tierney. More generally, if $X$ is a connected topological space, we show that the homotopy category of CW-complexes in Joyal-Tierney's model category of sheaves of sets on $X$ is equivalent to the homotopy category of groupoids. As an application of the ideas, we show that the algebraic $K$-theory groups of the category of pointed small categories are trivial, and more generally, the algebraic $K$-theory groups of any sufficiently "nice" Waldhausen category $\mathcal{A}$ of pointed small categories also vanishes, regardless of finiteness conditions assumed on the objects of $\mathcal{A}$. The vanishing of this $K$-theory implies that there is no nontrivial Euler characteristic defined on pointed small categories and satisfying certain niceness axioms.

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arXiv2016

How many adjunctions give rise to the same monad?

Given an adjoint pair of functors $F,G$, the composite $GF$ naturally gets the structure of a monad. The same monad may arise from many such adjoint pairs of functors, however. Can one describe all of the adjunctions giving rise to a given monad? In this paper we single out a class of adjunctions with especially good properties, and we develop methods for computing all such adjunctions, up to natural equivalence, which give rise to a given monad. To demonstrate these methods, we explicitly compute the finitary homological presentations of the free $A$-module monad on the category of sets, for $A$ a Dedekind domain. We also prove a criterion, reminiscent of Beck's monadicity theorem, for when there is essentially (in a precise sense) only a single adjunction that gives rise to a given monad.

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arXiv2015

Computation of the classifying ring of formal modules

In this paper, we develop general machinery for computing the classifying ring $L^A$ of one-dimensional formal $A$-modules, for various commutative rings $A$. We then apply the machinery to obtain calculations of $L^A$ for various number rings and cyclic group rings $A$. This includes the first full calculations of the ring $L^A$ in cases in which it fails to be a polynomial algebra. We also derive consequences for the solvability of some lifting and extension problems.

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