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The study of Manifolds having a complete Riemannian Metric.  
Riemannian geometry is a general space based on the Line Element
References
Besson, G.; Lohkamp, J.; Pansu, P.; and Petersen, P.  Riemannian Geometry.
  Providence, RI: Amer. Math. Soc., 1996.
 
Buser, P.  Geometry and Spectra of Compact Riemann Surfaces.  Boston, MA: Birkhäuser, 1992.
 
Chavel, I.  Eigenvalues in Riemannian Geometry.  New York: Academic Press, 1984.
 
Chavel, I.  Riemannian Geometry: A Modern Introduction.  New York: Cambridge University Press, 1994.
 
Chern, S.-S.  ``Finsler Geometry is Just Riemannian Geometry without the Quadratic Restriction.''
  Not. Amer. Math. Soc. 43, 959-963, 1996.
 
do Carmo, M. P.  Riemannian Geometry.  Boston, MA: Birkhäuser, 1992.