Hermann Minkowski  
   A brief biography with quotations and a bibliography.
   http://turnbull.dcs.st-and.ac.uk/~history/Mathematicians/Minkowski.html

Hendrik Lorentz  
   Brief biography.
   http://turnbull.dcs.st-and.ac.uk/%7ehistory/Mathematicians/Lorentz.html

Aleksandr Friedmann  
   A biography with reference to contemporaries, and a list of references.
   http://turnbull.dcs.st-and.ac.uk/~history/Mathematicians/Friedmann.html

Hermann Weyl  
   One of the first people to combine general relativity with the laws of electromagnetism
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Weyl.html

Marcel Grossmann  
   A brief biography on Marcel Grossmann, a classmate of Einstein who was the first to find a connection between tensor calculus and the theory of relativity.
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Grossmann.html

Georg Riemann  
   Although he died before the development of general relativity, his work in non-Euclidean geometries is very important to studying a curved spacetime.
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Riemann.html

Willem DeSitter  
   Mathematician who studied solution to general relativity and developed DeSitter space
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Sitter.html

Georgio Ricci-Curbastro  
   The mathematician who developed much of tensor calculus before Einstein found a use for it.
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Ricci-Curbastro.html

Luigi Bianchi  
   A mathematician who developed many theorems regarding Riemannian geometry
   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Bianchi.html

Karl Schwarzschild  
   The man who solved Einstein's equations and predicted the existence of black holes
   http://scienceworld.wolfram.com/biography/Schwarzschild.html