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Hayes, Andrew F.; Preacher, Kristopher J. . (2010). Quantifying and Testing Indirect Effects in Simple Mediation Models When the Constituent Paths Are Nonlinear. Multivariate Behavioral Research, 45(4), 627–660. doi:10.1080/00273171.2010.498290 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research