# Subgroups and subsets

I have some trouble with groups. Say we know that $A$ is a subgroup of $B$. If we have some subset of $A$, say $H$, can we deduce that $H$ is also a subgroup of $B$?

Thank you.

So if I have set of $2\times 2$ real matrices of form $\left( \begin{array}{cc} -a & b \\ b&a\end{array} \right)$ I only know its subset of $SL_2(\mathbb{R})$ but I have to show it's actually subgroup of $SL_2(\mathbb{R})$.

• You have to check that $H$ satisfies the group axioms. – N. F. Taussig Apr 26 '15 at 20:09

No, that's not true. In order to be a subgroup, it's necessary (but not sufficient) for a subset $H$ to contain the identity element (that is, the one usually denoted $0$, $1$, or $e$). So you can take any group $B$ you want, and then take any subgroup $A\subseteq B$ you want, and the subset $H=A\setminus\{e\}$ cannot be a subgroup of $B$.
No we cannot, for example if a subset consisting of exactly one element is never a subgroup, unless that element is $e$.
On the other hand if $H$ is a subgroup of $A$ and $A$ is a subgroup of $B$ then $H$ is a subgroup of $B$.
You probably mean $$K = \left\{ \begin{bmatrix} a & -b\\b & a\end{bmatrix} : \text{a, b not both zero} \right\},$$ and this is a subgroup, as $$\begin{bmatrix} a & -b\\b & a\end{bmatrix} \cdot \begin{bmatrix} c & -d\\d & c\end{bmatrix} = \begin{bmatrix} a c - b d & - (a d + b c) \\a d + b c & a c - b d\end{bmatrix} \in K,$$ and $$\begin{bmatrix} a & -b\\b & a\end{bmatrix}^{-1} = \frac{1}{a^2+ b^2} \begin{bmatrix} a & b\\-b & a\end{bmatrix} \in K.$$