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Jan Hennig

Publications and source records attributed to Jan Hennig.

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A cellular (co)homology computation for $\overline{M_{0,n}}$

In this article we set up and showcase cellular computations for (co)homology with values in strictly $\mathbb{A}^1$-invariant sheaves. These computations encapsulate many classical invariants like Chow groups and singular cohomology of the real points. They also extend enumerative arguments from algebraically closed fields to more general fields. The spaces considered here have to admit a cellular structure. Instead of using the classical notion of cellularity, i.e. having a stratification by affine spaces, more general stratifications by cohomologically trivial spaces are used, following Morel--Sawant. Examples of cellular spaces include projective spaces and their products, but also spaces such as $\overline{M_{0,n}}$, the moduli space of stable genus $0$ curves with $n$ marked points. For these examples, we showcase the computations and show how to derive the classical results. Hopefully, the following text provides enough evidence to be convincing that such computations are doable and is encouraging to start computing the cohomology for more cellular spaces. This is part of the author's PhD thesis.

math.AG

The real cycle class isomorphism for linear schemes

The real cycle class map $H^i(X,\underline{I}^j(\mathcal{L})) \rightarrow H^i_\text{sing}(X(\mathbb{R}),\mathbb{Z}(\mathcal{L}))$ is an isomorphism for $j\geq \dim(X)+1$ for any scheme $X$ over $\mathbb{R}$ by a result of Jacobson. It is also known to be an isomorphism for $j\geq i$, the earliest possible case, if $X$ is cellular due to Hornbostel-Wendt-Xie-Zibrowius. This paper generalizes their result to linear schemes, providing (precise) intermediate bounds on the range, where the real cycle class map is an isomorphism. Moreover, we show that Lerbet's conjectured upper bound for the exponent of the cokernel of $H^i(X,\underline{I}^i(\mathcal{L})) \rightarrow H^i_\text{sing}(X(\mathbb{R}),\mathbb{Z}(\mathcal{L}))$ cannot be improved. This is part of the author's PhD thesis.

math.AG