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Judd, B. R.. (1955). Operator Equivalents and Matrix Elements for the Excited States of Rare-Earth Ions. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 227(1171), 552–560. doi:10.1098/rspa.1955.0029 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research