Two-Step Subgroup Test

Theorem
Let $\left({G, \circ}\right)$ be a group.

Let $H$ be a subset of $G$.

Then $\left({H, \circ}\right)$ is a subgroup of $\left({G, \circ}\right)$ iff:


 * $(1): \quad H \ne \varnothing$, that is, $H$ is not empty
 * $(2): \quad a, b \in H \implies a \circ b \in H$
 * $(3): \quad a \in H \implies a^{-1} \in H$.

That is, $\left({H, \circ}\right)$ is a subgroup of $\left({G, \circ}\right)$ iff $\left({H, \circ}\right)$ is a $H$ be a nonempty subset of $G$ which is closed under its operation and closed under taking inverses.

Necessary Condition
Let $H$ be a subset of $G$ that fulfils the conditions given.

It is noted that the fact that $H$ is nonempty is one of the conditions.

It is also noted that the group product of $\left({H, \circ}\right)$ is the same as that for $\left({G, \circ}\right)$, that is, $\circ$.

So it remains to show that $\left({H, \circ}\right)$ is a group.

We check the four group axioms:

G0: Closure
The closure condition is given by condition $(2)$.

G1: Associativity
From Subset Product of Associative is Associative, associativity is inherited by $\left({H, \circ}\right)$ from $\left({G, \circ}\right)$.

G2: Identity
Let $e$ be the identity of $\left({G, \circ}\right)$.

From condition $(1)$, $H$ is not empty.

Therefore $\exists x \in H$.

From condition $(3)$, $\left({H, \circ}\right)$ is closed under taking inverses

Therefore $x^{-1} \in H$.

Since $\left({H, \circ}\right)$ is closed under $\circ$, $x \circ x^{-1} = e = x^{-1} \circ x \in H$.

G3: Inverses
From condition $(3)$, every element of $H$ has an inverse.

So $\left({H, \circ}\right)$ satisfies all the group axioms, and is therefore a group.

So by definition $\left({H, \circ}\right)$ is a subgroup of $\left({G, \circ}\right)$.

Sufficient Condition
Now suppose $\left({H, \circ}\right)$ is a subgroup of $\left({G, \circ}\right)$.


 * $(1): \quad H \le G \implies H \ne \varnothing$ from the fact that $H$ is a group and therefore can not be empty.
 * $(2): \quad a, b \in H \implies a \circ b \in H$ follows from group axiom G0 (Closure) as applied to the group $\left({H, \circ}\right)$.
 * $(3): \quad a \in H \implies a^{-1} \in H$ follows from group axiom G3 (Inverses) as applied to the group $\left({H, \circ}\right)$.

Comment
This is called the two-step subgroup test although, on the face of it, there are three steps to the test. This is because the fact that $H$ must be non-empty is usually an unspoken assumption, and is not specifically included as one of the tests to be made.

Some sources, for example, use this property of subgroups as the definition of a subgroup, and from it deduce that a subgroup is a subset which is a group.

Also see

 * One-Step Subgroup Test