Rips proved the conjecture of Morgan & Shalen (1991) that any finitely generated group acting freely on an R-tree is a free product of free abelian and surface groups (Bestvina & Feighn 1995).
Rips證明了Morgan & Shalen (1991)的猜想，就是每個自由作用在R-樹上的有限生成群都是自由阿貝爾群和曲面群的自由積（Bestvina & Feighn 1995）。