The expansion factors of an outer automorphism and its inverse
Abstract
A fully irreducible outer automorphism phi of the free group Fn of rank n has an expansion factor which often differs from the expansion factor of the inverse of phi. Nevertheless, we prove that the ratio between the logarithms of the expansion factors of phi and its inverse is bounded above by a constant depending only on the rank n. We also prove a more general theorem applying to an arbitrary outer automorphism of Fn and its inverse, and their entire spectrum of expansion factors.
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