Fermat's Little Theorem

Lemma: Let $p$ be a prime an $0 \leq k < p$ , then $$ {p\choose k} \equiv 0 \pmod p $$

Fermat’s Little Theorem

Let $p$ be a prime number, and $gcd (a, p) = 1$ , then

a^{p - 1} \equiv 1 (\mod p)

Cor: Let $p$ be a prime number, then for any $a$

a^{p} \equiv a (\mod p)

Lemma: If $p$ and $q$ are distinct primes with $a^{p} \equiv a (\mod q)$ and $a^{q} \equiv a (\mod p)$ , then

a^{p q} \equiv a (\mod p)

Def: If $n$ is a composite number is called pseudoprime if $n ∣ 2^{n} - 2$ . This is important because a Chinese mathematician thought that $n$ is prime iff $n ∣ 2^{n} - 2$ .

Th: If $n$ is an odd pseudoprime, then $M_{n} = 2^{n} - 1$ is a larger one.

Def: Expanding the definition of pseudoprimes, a composite number $n$ for which $a^{n} \equiv a (\mod n)$ is called a pseudoprime to the base $a$ , when $a = 2$ , it is just called a pseudoprime.

Def: If $n$ is a pseudoprime for all to every base $a$ , meaning $a^{n} \equiv a (\mod n)$ , are called an absolute pseudoprime or a Carmichael number.

Th: Let $n$ be a composite square-free integer, say $n = p_{1} p_{2} \dots p_{r}$ , where $p_{i}$ distinct primes. If $p_{i} - 1 ∣ n - 1$ , for $i = 1, 2, \dots, r$ , then $n$ is an absolute pseudoprime.