Math 2.0

definition

Subgroup

Definition

Let GG be a group with identity element ee. A subgroup of GG is a subset H⊆GH \subseteq G with these three properties. The identity ee is in HH. For all a,b∈Ha, b \in H, the product abab is in HH. For each a∈Ha \in H, the inverse a−1a^{-1} is in HH. The notation H≤GH \leq G means that HH is a subgroup of GG.

Remarks

A subgroup HH is a group with the operation of GG restricted to HH, and its identity is the identity of GG. Each group GG has the subgroups {e}\{e\} and GG. The even integers 2Z2\mathbb{Z} form a subgroup of (Z,+)(\mathbb{Z}, +). The natural numbers do not form a subgroup of (Z,+)(\mathbb{Z}, +), because −1-1 is not a natural number.

Explanations

L1 human

A subgroup is a smaller group that lives inside a bigger group and uses the same operation. To check that a subset is a subgroup, you check that it contains the identity and that it is closed under products and inverses. For example, the rotations of a square form a subgroup of all the symmetries of the square.

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Sources

  1. Web https://en.wikipedia.org/wiki/Subgroup, lead section
  2. Web https://en.wikipedia.org/wiki/Subgroup, Basic properties of subgroups