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Dense and empty BNSR-invariants of the McCool groups
- Part of
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- Journal:
- Canadian Journal of Mathematics , First View
- Published online by Cambridge University Press:
- 13 February 2026, pp. 1-27
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- Article
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An automorphism of the free group
$F_n$ is called pure symmetric if it sends each generator to a conjugate of itself. The group 
$\mathrm {PSAut}_n$ of all pure symmetric automorphisms and its quotient 
$\mathrm {PSOut}_n$ by the group of inner automorphisms are called the McCool groups. In this article, we prove that every BNSR-invariant 
$\Sigma ^m$ of a McCool group is either dense or empty in the character sphere, and we characterize precisely when each situation occurs. Our techniques involve understanding higher generation properties of abelian subgroups of McCool groups, coming from the McCullough–Miller space. We also investigate further properties of the second invariant 
$\Sigma ^2$ for McCool groups using a general criterion due to Meinert for a character to lie in 
$\Sigma ^2$.
