1. P. Alestado, H. Helling, On torus fibrations over the circle, preprint E97-005, SFB 343, Bielefeld, 1997.
2. J. O. Button, Invariant trace fields of once-punctured torus bundles, Kodai Math. J. 28 (2005), no. 1, 181–190.
3. H. Helling, The trace field of a series of hyperbolic manifolds, preprint 99-072, SFB 343, Bielefeld, 1999.
4. H. Helling, Ch. Menzel, Towards an algebraic construction of the hyperbolic structure on surface bundles, preprint 99-074, SFB 343, Bielefeld, 1999.
5. H. Helling, The Markov group, unpublished manuscript, Bielefeld, 2013.