Automorphisms of extremal unimodular lattices in dimension 72

The paper narrows down the possible automorphisms of extremal even unimodular lattices of dimension 72. With extensive computations in {\sc Magma} using the very sophisticated algorithm for computing class groups of algebraic number fields written by Steve Donnelly it is shown that the extremal even unimodular lattice $Γ_{72}$ from \cite{dim72} is the unique extremal even unimodular lattice of dimension 72 that admits a large automorphism, where a $d\times d$ matrix is called large, if its minimal polynomial has an irreducible factor of degree $> d/2$.

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