Einstein and Tensor Calculus

http://www.nytimes.com/2015/11/24/science/a-century-ago-einsteins-theory-of-relativity-changed-everything.html

What a shame that Dennis Overbye's story about the General Theory of Relativity didn't mention the mathematical foundation for Einstein's work. The General Theory could not have been developed without Tensor Calculus, which makes possible modeling of complex motion in uneven space. Tenor calculus is so difficult even Einstein needed a tutor, his friend mathematician Marcel Grossman, "Between the years of 1915 to 1919, Einstein held a correspondence with the Italian mathematician Tullio Levi-Civita – who in 1900 published perhaps the most important work on tensor calculus to this very day - who desired to help him [Einstein] fix some mathematical errors he had found in Einstein’s work."