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Some comments on 6d global gauge anomalies

Global gauge anomalies in $6d$ associated with non-trivial homotopy groups $π_6(G)$ for $G=SU(2)$, $SU(3)$, and $G_2$ were computed and utilized in the past. In the modern bordism point of view of anomalies, however, they come from the bordism groups $Ω^\text{spin}_7(BG)$, which are in fact trivial and therefore preclude their existence. Instead, it was noticed that a proper treatment of the $6d$ Green-Schwarz mechanism reproduces the same anomaly cancellation conditions derived from $π_6(G)$. In this paper, we revisit and clarify the relation between these two different approaches.