On the Witten--Reshetikhin--Turaev invariants of torus bundles

By methods similar to those used by Lisa Jeffrey, we compute the quantum $\mathrm{S}\mathrm{U}(N)$-invariants for mapping tori of trace $2$ homeomorphisms of a genus $1$ surface when $N=2,3$ and discuss their asymptotics. In particular, we obtain directly a proof of a version of Witten’s asymptotic expansion conjecture for these 3-manifolds. We further prove the growth rate conjecture for these 3-manifolds in the $\mathrm{S}\mathrm{U}(2)$ case, where we also allow the 3-manifolds to contain certain knots. In this case we also discuss trace $-2$ homeomorphisms, obtaining – in combination with Jeffrey’s results – a proof of the asymptotic expansion conjecture for all torus bundles.