* Samuel Eilenberg & Tudor Ganea, On the Lusternik-Schnirelmann category of abstract groups, Annals of Mathematics, 2nd Ser., 65 ( 1957 ), no.
* Samuel Eilenberg, Tudor Ganea, On the Lusternik-Schnirelmann category of abstract groups, Annals of Mathematics, 2nd Ser., 65 ( 1957 ), no.
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