When proving that every subgroup of a cyclic group is cyclic.
Let $G = \langle a \rangle$ and suppose that $H$ is a subgroup of $G$ and assume that $H \ne \{e\} $.
The author begins with the claim that $H$ contains an element of the form $a^{t}$, where $t$ is positive.
To verify this claim, he says,
Since $G = \langle a \rangle$, every element of H has the form $a^{t}$; and when $a^{t}$ belongs to $H$ with $t<0$, then $a^{-t}$ belongs to $H$ also and -t is positive.
I see the first part clearly, but I am not able to see clearly what the second part which begin after ';' is saying.
Best Answer
It's easy, suppose you choose an arbitrary element $p$ from the subgroup $H$. Then as the elements of $H$ are also the elements of the group $G=<a>$ then p is of the form $a^t$ for some $t\in \mathbb{Z}$. If this $t$ is positive then we are done but if $t\lt0$ then H being the subgroup $a^{-t} \in H$ and $-t\gt0$. So in any case we have $a^s \in H$ for $s\gt0$.