Ph. Hall, G. Higman, “On the $p$-length of $p$-soluble groups and reduction theorems for Burnside's problem”, Matematika, 13:2 (1969), 64–104; Proc. London Math. Soc., 6 (1956), 1–42
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2.
Philip Hall, “Nilpotent groups”, Matematika, 12:1 (1968), 3–36
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