Critical points of the distance function
Hessian and Laplacian comparison theorems
Toponogov's theorem
Bishop Volume comparison and Gromov's relative
volume comparison Theorems
Cheeger-Gromoll Splitting Theorem
Gromov-Hausdorff distance
Cheeger's Finiteness Theorem
REFERENCES:
Riemannian
Geometry, A Modern Introduction by Isaac Chavel
(Cambridge University
Press 1995)
Comparison
theorems in Riemannian geometry by Jeff Cheeger and David Ebin
(North-Holland
1975)
Lectures
on differential geometry by Richard Schoen and Shing-Tung Yau
(International
Press 1994)