TITLE: Why doesn't Fermat's Little theorem work for 4 and 9?
QUESTION [11 upvotes]: 4 and 9 are relatively prime, but $4^8$ = 65536, which is not $1 \mod 9$. I can't figure out why because I've been told that $a^{p-1}$ is equivalent to $1 \mod p$ if $p$ and $a$ are relatively prime...so...what am I missing?

REPLY [16 votes]: You can't apply Fermat's theorem: $$a^{p-1} \equiv 1 \pmod p$$
because $9$ is not a prime.
However,you can use Euler's theorem:
$$a^{\phi(m)} \equiv 1 \pmod m$$
$$a=4$$
$$m=9$$
$$(a,m)=1$$
$$\phi(9)=\phi(3^2)=3^2 \left (1-\frac{1}{3} \right )=9 \frac{2}{3}=6$$
Therefore:
$$4^6 \equiv 1 \pmod 9$$

REPLY [7 votes]: Fermat's Little Theorem is true only if $p$ is prime - the statement is  $$a^p \equiv a \mod p$$ for $p$ prime. When $(a,p) = 1$, this is equivalent to the usual formulation of the theorem (we can divide by $a$): $$a^{p-1} \equiv 1 \mod p$$ if $p$ is prime and $(a,p) = 1$.  In this case, $9$ is not prime, so the theorem will fail.

There is however a generalisation that works for all integers, usually called the Fermat-Euler Theorem: we define $\phi(n)$ to be the number of integers less than $n$ that are coprime to $n$. Then if $(a,n)=1$, $$a^{\phi(n)}  \equiv 1 \mod n$$
We have $\phi(9) = 6$ and indeed, $4^6 \equiv 1 \mod 9$.