e d (p~ (s ), Pt (s )) 2: (( 1 - I) e - E) t for all t, t l sufficiently large. In particular, up to the term Et (and E can be chosen arbitrarily small), the concatenation of the geodesic from x to pds) with the geodesic from Pt' (s) to Pt (s) is a shortest connection from x to Pt (s). By comparison with Euclidean geometry we conclude that for any R 2: 0, the distance d(O's,t(R),O's,dR)) is arbitrarily small for all t, t l sufficiently large.

4) still hold and b(x, y, y) = 0 for all x E X and y E X. 5 Remarks. (a) If X is locally compact, then X and X(oo) are compact. For this note that we can apply the Theorem of Arzela-Ascoli since by (x) is normalized by by (x) (y) = 0 and since by (x) has Lipschitz constant 1 for all x EX. (b) In potential theory, one uses Green's functions G(x,y) to define the Martin boundary in an analogous way. Instead of using differences b(x, y, z) = d(x,z) - d(x,y), one takes quotients K(x,y,z) = G(x,z)/G(x,y).

X be a unit speed geodesic as(t) = Pt(s) and as(oo) = a(oo). Then clearly f(a) = f(a s ) = Lo-s(t)(((a), a 8 (00)). Therefore ptl [s, 00) and as I [t, 00) span a flat cone Gs,t. This cone contains aUt, 00)) if s < O. Now X is locally compact. Hence we obtain a flat plane containing a as a limit of the cones GSn,t n for some appropriate sequences Sn ----+ -00 and t n ----+ -00. 6 Problem. Suppose X is geodesically complete and irreducible with a group r of isometries satisfying the duality condition.

### Connections, curvature, and cohomology by Werner Hildbert Greub, Stephen Halperin, James Van Stone

by James

4.5