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Speck, M.; Buchholz, J.; Sellier, M. . (2014). A mathematical model of a twin ducted-fan vertical takeoff and landing jetpack. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, 228(10), 1831–1844. doi:10.1177/0954410013503060 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research