We give a description of the boundary of a complex of free factors that is analogous to E. Klarreich’s description of the boundary of a curve complex. The argument uses the geometry of folding paths developed by Bestvina and Feighn and the structure theory of trees on the boundary of Outer space developed recently by Coulbois, Hilion, Lustig, and Reynolds.
"The boundary of the complex of free factors." Duke Math. J. 164 (11) 2213 - 2251, 15 August 2015. https://doi.org/10.1215/00127094-3129702