I also got involved with **geodesics on surfaces.** I gave the first
examples of compact surfaces with focal points but no **conjugate
points** [12]. Leon Green and I showed that a compactly
supported perturbation of the Euclidean plane which has no
conjugate points is isometric to the plane [24]. This work was
later extended by Chris Croke.

[12]. On the Variety of Manifolds without Conjugate Points, *Trans.
Amer. Math. Soc.*** 210,** 185-201 (1975).

[24]. Planes without Conjugate Points, (with Leon Green), *J.
Differential Geom.*** 22,** 43-47 (1985).