Kiyota–Murai–Wada rationality conjecture for Cartan matrix eigenvalues

Let BB be a block of a finite group GG over an algebraically closed field of characteristic p>0p>0, let CZl×lC\in\mathbb{Z}^{l\times l} be its Cartan matrix, and let DD be a defect group of BB. Write EDED for the multiset of elementary divisors of CC, EVEV for the multiset of eigenvalues of CC, and let ρ(C)\rho(C) denote the spectral radius of CC. Kiyota–Murai–Wada conjecture. The following assertions are equivalent: EV=EDEV=ED; ρ(C)=D\rho(C)=|D|; ρ(C)Z\rho(C)\in\mathbb{Z}; and EVZEV\subseteq\mathbb{Z}. The conjecture is known for blocks of finite or tame representation type, for pp-solvable groups, and when DGD\unlhd G, but the stated equivalence is false in general: the principal 55-block of PSU(3,4)\operatorname{PSU}(3,4) has ρ(C)=D\rho(C)=|D| while EVEDEV\ne ED.

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Primary source

Benjamin Sambale, “A counterexample to a conjecture of Kiyota, Murai and Wada”, arXiv:1705.01321 (2017).

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