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small in 4.5
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UlrikBuchholtz committed Aug 8, 2024
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Expand Up @@ -1844,9 +1844,9 @@ \section{The sign homomorphism}
$\Aut(A)$ to $\SG_2$ for each $A$, since it does so for $A \jdeq
\sh_{\SG_n}$. Even better, this abstract homomorphism comes from a concrete one
$\sgn^A : \Hom(\Aut(A),\SG_2)$ for each finite set $A$. Indeed, since
$T \eqto U$ is a 2-element set for any 2-element sets $T$ and $U$, we can
consider the map $\Bsgn^A_\div : \BAut(A) \to \BSG_2$ that associates
$B : \BAut(A)$ with $(\Bsgn_\div(A) \eqto \Bsgn_\div(B))$. The identification of
$T \eqto U$ is a $2$-element set for any $2$-element sets $T$ and $U$, we can
consider the map $\Bsgn^A_\div : \BAut(A) \to \BSG_2$ that maps
$B : \BAut(A)$ to $(\Bsgn_\div(A) \eqto \Bsgn_\div(B))$. The identification of
$\Bsgn^A_\div(A)$ with $\{\pm1\}$ mentioned above makes $\Bsgn^A_\div$ into a
pointed map $\Bsgn^A : \BAut(A) \ptdto \BSG_2$, i.e., it defines an homomorphism
$\sgn^A : \Hom(\Aut(A),\SG_2)$, as announced.%
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