A [[subset]] ''H'' of a [[group]] (''G'',<math>\cdot</math>) is called a '''subgroup''', if (''H'',<math>\cdot</math>) is itself a group.
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A [[subset]] ''H'' of a [[Group (mathematics)|group]] (''G'',<math>\cdot</math>) is called a '''subgroup''', if (''H'',<math>\cdot</math>) is itself a group.
==Simple Examples==
==Simple Examples==
# <math>(2\mathbb{Z},+)</math> is a subgroup of <math>(\mathbb{Z},+) </math>
# <math>(2\mathbb{Z},+)</math> is a subgroup of <math>(\mathbb{Z},+) </math>