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Bock, Hans Georg; Kostina, Ekaterina; Kostyukova, Olga . (2007). Covariance Matrices for Parameter Estimates of Constrained Parameter Estimation Problems. SIAM Journal on Matrix Analysis and Applications, 29(2), 626–642. doi:10.1137/040617893 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research