Dehn fillings of knot manifolds containing essential twice-punctured tori

Abstract

We show that if a hyperbolic knot manifold $M$ contains an essential twice-punctured torus $F$ with boundary slope $\beta$ and admits a filling with slope $\alpha$ producing a Seifert fibred space, then the distance between the slopes $\alpha$ and $\beta$ is less than or equal to $5$ unless $M$ is the exterior of the figure eight knot. The result is sharp; the bound of $5$ can be realized on infinitely many hyperbolic knot manifolds. We also determine distance bounds in the case that the fundamental group of the $\alpha$-filling contains no non-abelian free group. The proofs are divided into the four cases $F$ is a semi-fibre, $F$ is a fibre, $F$ is non-separating but not a fibre, and $F$ is separating but not a semi-fibre, and we obtain refined bounds in each case.