A tensor notation which considers the Riemann tensor 
as a matrix 
with indices 
and 
.
Petrov Notation
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References
Weinberg, S. Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity. New York: Wiley, p. 142, 1972.Referenced on Wolfram|Alpha
Petrov NotationCite this as:
Weisstein, Eric W. "Petrov Notation." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PetrovNotation.html