1. M. Bestvina, Degenerations of the hyperbolic space, Duke Mathematical Journal 56 (1988), 143–161.
2. M. Bestvina and M. Feighn, Notes on Sela’s work: Limit groups and Makanin–Razborov diagrams, in Geometric and Cohomological Methods in Group Theory, London Mathematical Society Lecture Note Series, Vol. 358, Cambridge University Press, 2009, pp. 1–29.
3. V. Guirardel, Limit groups and groups acting freely on Rn-trees, Geometry and Topology 8 (2004), 1427–1470.
4. V. Guirardel, Actions of finitely generated groups on R-trees, Université de Grenoble. Annales de l’Institut Fourier 58 (2008), 159–211.
5. H. J. Keisler, Ultraproducts which are not saturated, Journal of of Symbolic Logic (1967), 23–46.