A subgroup $S$ of a (typically abelian) group $G$ is said to be essential if whenever $H$ is a non-trivial subgroup of $G$, the intersection of $S$ and $H$ is non-trivial: here "non-trivial" means "containing an element other than the identity".
- Phillip A. Griffith, Infinite Abelian group theory, Chicago Lectures in Mathematics (University of Chicago Press, 1970) ISBN 0-226-30870-7 p.19
Essential subgroup. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Essential_subgroup&oldid=30521