arXiv subjects

Bjørnar Gullikstad Hem

Publications and source records attributed to Bjørnar Gullikstad Hem.

4 recordsLinked to original sources

arXiv2026

Cross effects for functors from posets

We establish a precise relationship between functor calculus and the projective dimension of multipersistence modules. Specifically, we develop a new notion of functor calculus for functors from posets, which detects vanishing total fibers of cubes. We give an explicit construction of the universal approximation functors of this functor calculus. We then use these approximations to prove two new theorems, providing necessary and sufficient conditions for an $n$-parameter multipersistence module to have projective dimension at most $n-1$ and at most $n-2$.

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arXiv2025

Poset functor cocalculus and applications to topological data analysis

We introduce a new flavor of functor cocalculus, called \emph{poset cocalculus}, as a tool for studying approximations in topological data analysis. Given a functor from a distributive lattice to a model category, poset cocalculus produces a Taylor telescope of codegree $n$ approximations of the functor, where a codegree $n$ functor takes strongly bicartesian $(n+1)$--cubes to homotopy cocartesian $(n+1)$--cubes. We give several applications of this new functor cocalculus. We prove that the codegree $n$ approximation of a multipersistence module is stable under an appropriate notion of interleaving distance. We draw connections to filtrations of simplicial complexes, and show that the Vietoris-Rips filtration is precisely the codegree 2 approximation of the Čech filtration. We demonstrate that the codegree 1 approximation of the space of simplicial maps between two simplicial complexes is in some sense the space of continuous maps between their realizations, and that this statement can be made precise.

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arXiv2025

Decomposing multipersistence modules using functor calculus

We apply poset cocalculus, a functor calculus framework for functors out of a poset, to study the problem of decomposing multipersistence modules into simpler components. We both prove new results in this topic and offer a new perspective on already established results. In particular, we show that a pointwise finite-dimensional bipersistence module is middle-exact if and only if it is isomorphic to the homology of a homotopy degree 1 functor, from which we deduce a novel, more synthetic proof of the interval decomposability of middle-exact bipersistence modules. We also give a new decomposition theorem for middle-exact multipersistence modules indexed over a finite poset, stating that such a module can always be written as a direct sum of a projective module, an injective module, and a bidegree 1 module, even in the case where it is not pointwise finite-dimensional.

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arXiv2025

The discrete flow category: structure and computation

In this article, we use concepts and methods from the theory of simplicial sets to study discrete Morse theory. We focus on the discrete flow category introduced by Vidit Nanda, and investigate its properties in the case where it is defined from a discrete Morse function on a regular CW complex. We design an algorithm to efficiently compute the Hom posets of the discrete flow category in this case. Furthermore, we show that in the special case where the discrete Morse function is defined on a simplicial complex, then each Hom poset has the structure of a face poset of a regular CW complex. Finally, we prove that the spectral sequence associated to the double nerve of the discrete flow category collapses on page 2.

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