Fibration Categories are Fibrant Relative Categories

A relative category is a category with a chosen class of weak equivalences. Barwick and Kan produced a model structure on the category of all relative categories, which is Quillen equivalent to the Joyal model structure on simplicial sets and the Rezk model structure on simplicial spaces. We will prove that the underlying relative category of a model category or even a fibration category is fibrant in the Barwick--Kan model structure.Comment: 21 pages; comments welcom

Topological modular forms with level structure: decompositions and duality

Topological modular forms with level structure were introduced in full generality by Hill and Lawson. We will show that these decompose additively in many cases into a few simple pieces and give an application to equivariant $TMF$. Furthermore, we show which $Tmf_1(n)$ are self-Anderson dual up to a shift, both with and without their natural $C_2$-action.Comment: Rewritten introduction. Minor updates, corrections and additons. 54 pages. Comments welcom

Additive decompositions for rings of modular forms

We study rings of integral modular forms for congruence subgroups as modules over the ring of integral modular forms for the full modular group. In many cases these modules are free or decompose at least into well-understood pieces. We apply this to characterize which rings of modular forms are Cohen--Macaulay and to prove finite generation results. These theorems are based on decomposition results about vector bundles on the compactified moduli stack of elliptic curves.Comment: Rewritten introduction, updated references. This article supersedes the algebraic part of arXiv:1609.0926

Additive decompositions for rings of modular forms

We study rings of integral modular forms for congruence subgroups as modules over the ring of integral modular forms for the full modular group. In many cases these modules are free or decompose at least into well-understood pieces. We apply this to characterize which rings of modular forms are Cohen--Macaulay and to prove finite generation results. These theorems are based on decomposition results about vector bundles on the compactified moduli stack of elliptic curves.Comment: Minor changes + new appendix. This article supersedes the algebraic part of arXiv:1609.0926

Fibrancy of Partial Model Categories

We investigate fibrancy conditions in the Thomason model structure on the category of small categories. In particular, we show that the category of weak equivalences of a partial model category is fibrant. Furthermore, we describe connections to calculi of fractions.Comment: 30 page

A Whitehead theorem for periodic homotopy groups

We show that $v_n$-periodic homotopy groups detect homotopy equivalences between simply-connected finite CW-complexes

Connective Models for Topological Modular Forms of Level nn

The goal of this article is to construct and study connective versions of topological modular forms of higher level like $\mathrm{tmf}_1(n)$. In particular, we use them to realize Hirzebruch's level-$n$ genus as a map of ring spectra.Comment: 27 pages; v2: added several clarifications and minor correcionts in response to referee's comments, final version to appear in AG