The toral rank and Carlsson conjectures for manifolds

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Let MM be a closed connected manifold. Its toral rank is the largest integer rr such that TrT^r acts almost freely on MM, and, for a prime pp, its pp-rank is the largest integer rr such that (Z/p)r(\mathbb{Z}/p)^r acts freely on MM.

Toral rank and Carlsson conjectures.

dim⁡H∗(M,Q)≥2rank⁡(M)\dim H^*(M,\mathbb{Q})\geq 2^{\operatorname{rank}(M)}

and, for every prime pp,

dim⁡H∗(M,Z/p)≥2rank⁡p(M).\dim H^*(M,\mathbb{Z}/p)\geq 2^{\operatorname{rank}_p(M)}.

These conjectures, attributed respectively to S. Halperin and G. Carlsson, relate the ranks of torus and elementary abelian group actions to the total cohomology of the manifold. Their resolution status is not specified in the source.

References

Primary source

Jordi Daura Serrano, “Large and iterated finite group actions on manifolds admitting non-zero degree maps to nilmanifolds”, arXiv:2506.11174 (2025).

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