I am not a number theorist and I am learning about relations.
I encountered a relation that says
$a \leq b$ if $a$ divides $b$
Can someone clarify what it means to a number to divide another number?
Does it mean what I think? $a$ divides $b$ if $a | b \in \mathbb{Z}$?
$\endgroup$ 7So given a set $S = \{a, a^2, a^3, \ldots\}$, with relation $a | b \leftrightarrow a \leq b$, does the relation hold going from left to right or right to left? i.e. $a|a^2, a^2|a^3, \ldots$
2 Answers
$\begingroup$Given two integers $a$ and $b$, we say $a$ divides $b$ if there is an integer $c$ such that $b=ac$.
Source.
This is what $a$ divides $b$ means. The shorthand notation is $$a|b$$.
In your example, $$a|a^2\iff a\leq a^2$$ since by definition there exists $c$ such that $a^2 = ac$, namely $a = c$.
$\endgroup$ 6 $\begingroup$We say $a$ divides $b$, denoted by $a | b$, if $b$ is a multiple of $a$ (ie, $b$ is an integer multiple of $a$). Equivalently, $a |b$ iff $b=ka$ for some integer $k$.
To remember what "$2$ divides $6$" means, perhaps you can remember the phrase "$2$ divides $6$ into $3$ parts". Hence, $2 | 6$.
Note that $2 | 0$ because $0$ is an integer multiple of $2$: $0 = k2$ for some integer $k$. Just take $k=0$.
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