Blockwise spectrum problem for integral group rings

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Let GG be a finite group. Let V(ZG)V({\mathbb Z}G) be the normalized unit group of ZG{\mathbb Z}G, let PCI⁡(QG)\operatorname{PCI}({\mathbb Q}G) be the set of primitive central idempotents of QG{\mathbb Q}G, and let spec⁡(A)\operatorname{spec}(A) denote the spectrum of a finite group or its corresponding block. For e∈PCI⁡(QG)e\in\operatorname{PCI}({\mathbb Q}G), write GeGe and V(ZG)eV({\mathbb Z}G)e for their projections to the block determined by ee.

Blockwise Spectrum Problem. For every primitive central idempotent e∈PCI⁡(QG)e\in\operatorname{PCI}({\mathbb Q}G),

spec⁡(Ge)=spec⁡(V(ZG)e).\operatorname{spec}(Ge)=\operatorname{spec}(V({\mathbb Z}G)e).

This is a blockwise weakening of the spectrum problem: it asks for equality of the spectra after projection to each primitive central block, rather than only a global statement.

References

Primary source

Robynn Corveleyn, Geoffrey Janssens and Doryan Temmerman, “Representing in Low Rank I: conjugacy, topological and homological aspects”, arXiv:2512.22052 (2026).

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