# In number theory, is it always true that..

Note: $\text{ord}_ma = k$ here is the smallest $k$ such that $a^k \equiv 1 \pmod m$, not the highest power of $m$ that divides $a$.

Is it always that case that if $\text{ord}_ma=k$ and if $\text{ord}_mb=l$ then $\text{ord}_m(ab)=kl$ should be the $\text{lcm}(k,l)$ of which is true if $\gcd(k,l)=1$?

How would I prove this?

Perhaps I could consider the case where $b$ is a multiplicative inverse of $a$?

So, $ab \equiv 1\pmod{m}$

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This seems false in general. $\mathrm{ord}_5(3) = \mathrm{ord}_5(2) = 4$ but $\mathrm{ord}_5(2\cdot 3) = 1$. –  EuYu Oct 25 '12 at 15:23
@EuYu This isn't what I said. The example you used has the order being the same, $4$, but I need the order to be different. Perhaps, I read it wrong and it should be $\gcd(a,b)=1$. Maybe that is what's giving me so much trouble. –  student.llama Oct 25 '12 at 15:46
Could you clarify your question? Especially as your idea of $b=a^{-1}$ leads to order 0. –  Hagen von Eitzen Oct 25 '12 at 16:03

Lemma. Let $x$ and $y$ be commuting elements of a group $G$, and assume $x$ and $y$ are of finite orders $n$ and $m$, respectively. Suppose that for every prime $p$ that divides $nm$, the largest power of $p$ that divides $n$ is different from the largest power of $p$ that divides $m$. Then the order of $xy$ in $G$ is $\mathrm{lcm}(n,m)$.
Proof. Let $\ell=\mathrm{lcm}(n,m)$. It is easy to verify that $(xy)^{\ell}=1$, so we just need to show that $\ell$ is the smallest positive integer $k$ such that $(xy)^k=1$. Suppose that $(xy)^k = 1$, with $0\lt k\leq \ell$.
Since $(xy)^k = x^ky^k = 1$, then $x^k = y^{-k}$, and in particular $x^k$ has the same order as $y^k$. The order of $x^k$ is $n/\gcd(n,k)$, and the order of $y^k$ is $m/\gcd(n,k)$. Let $p$ be a prime that divides $\ell$, and let $a=\mathrm{ord}_p(n)$, $b=\mathrm{ord}_p(m)$, and $c=\mathrm{ord}_p(k)$. Assume $b\lt a$. If $c\lt a$, then $\mathrm{ord}_p(n/\gcd(n,k)) = a-c$, and $\mathrm{ord}_p(m/\gcd(m,k))=\max(0,b-c)\lt a-c$, which is impossible. Thus, $c=a$. A symmetric argument shows that if $a\lt b$, then $c=b$. That is, for all primes $p$ that divide $\ell$, we have $\mathrm{ord}_p(k) = \max(\mathrm{ord}_p(n),\mathrm{ord}_p(n)) = \mathrm{ord}_p(\ell)$. Hence $k=\ell$. QED