# Modular arithmetic power rule implications

Assume that $a \equiv b \text{ (mod m)}$, the following: $$a^n\equiv b^n \text{ (mod m)}$$ is true.

However, does $a^n\equiv b^n \text{ (mod m)}$ imply that $a \equiv b \text{ (mod m)}$ is true when n is odd?

If n is odd, it seems like this is justifiable because negative numbers do not become positive numbers if raised to an odd power.

Also would it be true for when n is even if say $a^n\equiv b^n \text{ (mod m)}$ imply $a \equiv ±b \text{ (mod m)}$?

• No, its is not true. For example, if $m=7$ and $n=3$, then $2^3\equiv 1^3 \pmod{7}$, but $2\not{\equiv} 1 \pmod{7}$. – xarles Jul 29 '18 at 6:52

The above example by Lord Shark the Unknown indicates that no complete answer can be found. But sometimes, we do have that kind of higher cancellation:

We have, by telescopic sum, $$a^n - b^n = (a - b) \sum_{j=0}^{n-1} a^{n-1-j}b^j,$$ so that $a^n \equiv b^n \mod m$ implies $a \equiv b \mod m$ if $$\sum_{j=0}^{n-1} a^{n-1-j}b^j$$ is not a zero divisor.

$2^3\equiv4^3\pmod 7$, but $2\not\equiv4\pmod7$.

First, all you say about being positive or negative modulo $n$ makes really no sense, since $a\equiv a-m \pmod{m}$, so any positive number is equivalent to a negative number and viceversa.

However, there is a natural condition to put on $n$ related to $m$, if $m$ is prime: if $n$ and $m-1$ are coprime (so they share no comon divisor) , then $a^n\equiv b^n \pmod{m}$ implies $a \equiv b \pmod{m}$.

• Is there a reason they have to be coprime? – LHC2012 Jul 29 '18 at 7:08
• See my posting for a counterexample to your final claim. – Lord Shark the Unknown Jul 29 '18 at 7:11
• Ups, I forgot to write the $\varphi$ in front of $m$... – xarles Jul 29 '18 at 7:16
• OK, then $2^5\equiv 6^5\pmod 8$ and $2\not\equiv6\pmod 8$. Here $n=5$ and $\phi(m)=4$ are coprime. – Lord Shark the Unknown Jul 29 '18 at 7:25
• Sorry for the errors, now stated in the prime modulus case. If $m$ is not prime, the result is true for the invertible elements modulo $m$, changing $m-1$ by Euler's $\varphi$ function. – xarles Jul 29 '18 at 7:35

For $\gcd(n,\phi(m))=1$, then you will have $a^n\equiv b^n \implies a\equiv b \bmod m.$ (Since $\phi(m)$ is always even for $m>2$, $n$ will always be odd under this condition, but many odd numbers will not fulfill this).

When $\gcd(n,\phi(m))>1$, you can have two (or more) different values producing the same $n$th power - for example, $4^3\equiv 9^3\equiv 29 \bmod 35$.