Existence of Prime between Prime and Factorial

Theorem
Let $p$ be a prime number.

Then there exists a prime number $q$ such that:
 * $p < q \le p! + 1$

where $p!$ denotes the factorial of $p$.

Proof
Let $N = p! + 1$.

If $N$ is prime, then the proof is complete.

Otherwise, from Positive Integer Greater than 1 has Prime Divisor, $N$ has a prime divisor $q$.

From Integer Absolute Value Greater than Divisors:
 * $q < N$

Aiming for a contradiction, suppose $q \le p$.

Then by the definition of factorial:
 * $q \mathop \backslash p!$

But $q$ was chosen so that :$q \mathop \backslash N$.

Let $p! = q m_1, N = q m_2$.

Then:
 * $1 = N - p! = q \left({m_2 - m_1}\right)$

and so:
 * $q \mathop \backslash 1$

From Integer Absolute Value Greater than Divisors:
 * $q \le 1$

and so $q$ cannot be a prime number.

It follows by Proof by Contradiction that $q > p$.

Also see

 * Euclid's Theorem, of which this is a variant.