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Prof. Dr. Hans-Dieter Martin; Mirjana Eckert-Maksić; Dipl.-Chem. Bernhard Mayer. (1980). Diagonal Interaction in the Four-Membered Ring. Experimental Demonstration of a Dominating Circumannular Conjugation: 5,6-Dimethylenebicyclo[2.1.1]hexane. , 19(10), 807–809. doi:10.1002/anie.198008071 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research