# Prove that the nth prime number $p_n$ satisfies $p_n\leq 2^{2n-1}$

Prove that the nth prime number $p_n$ (with $p_1 = 2, p_2 = 3, p_3 = 5$, etc.) satisfies $p_n \leq 2^{2n-1}$

So far I have figured out that $p_n$ = the nth prime and that I have to use mathematical induction to prove $p_n \leq 2^{2n-1}$. This is similar to the proof of infinitely many primes such that
$m = 1 + p_1 p_2 p_3\cdots p_n$ so there exists a prime $p | m$.

With this information I have concluded that $p_{n+1} \leq p \leq m = 1 + p_1 p_2 p_3\cdots p_n$

I need to figure out how to produce the right side of this inequality with induction. I'm not sure how to do this.

Thank you for any and all help.

• Good so far. What can you say about that product? Use the induction hypothesis. – saulspatz Feb 15 '18 at 4:17
• For induction purposes, if you can show that $p_{k+1}\le 4p_k$, you are good. Not saying that is easy, but that is the induction frame. you need to meet. – Joffan Feb 15 '18 at 4:17
• Your bound would have to be a lot better than $1+p_1p_2p_3\ldots p_n$ for this to work. – Robert Israel Feb 15 '18 at 4:57

A theorem(I forgot its name) states that there is always a prime between $n$ and $2n$.

Assume $$p_n \le 2^{2n-1}$$ is true.

Since there is a prime between $p_n$ and $2p_n$, $p_{n+1}<2p_n$.

So, $$p_{n+1}<2*2^{2n-1}<2^{2n}<2^{2(n+1)-1}$$

Therefore, $P(n+1)$ is true when $P(n)$ is true.

The base case is $n=1$, $p_1=2$.

By the principle of mathematical induction, the statement is true.

• I guess OP is asking for a proof from scratch in which case, using a Bertrand's Postulate imply proving the theorem first. – Nilos Feb 15 '18 at 8:54