Mathematics, Physics, Einstein, Relativity, Tensor.
obtained by projecting onto the normal lines to the coordinate hyperplanes. [1] References [1].https://www.wikiwand.com/en/Covariance_and_contravariance_of_vectors [2]. https://www.grc.nasa.gov/www/k 12/Numbers/Math/documents/Tensors_TM2002211716.pdf