sage: f = train_track.FreeGroupAutomorphism('a->cba,b->baa,c->cbac')
sage: f.inverse().train_track().expansion_factor()
/home/vincent/programming/sage-train-track/train_track/graph_self_map.py:342: DeprecationWarning: non-integer arguments to randrange() have been deprecated since Python 3.10 and will be removed in a subsequent version
  eigenvalues = self.matrix().charpoly().roots(AA)
3.302775637731995?