Is there always a prime between $n$ and $2n$? if we are interested to seek for the numbers of primes between $1-100$ and $100-1000$ or 1000..., why we don't asked if there is a always a prime between $n$ and $2n$ mayeb this interesting question help us to predict the numbers of primes between $1-100$,or $100-1000 $ or $1000-..$?
note: $n$ is natural number $ > 1$
I would be interest for any replies or comments .Thank you 
 A: $$\begin{array}{l}\text{Chebyshev said it,}\cr
\text{And I say it again,}\cr
\text{There is always a prime}\cr
\text{Between n and 2n.}\end{array}$$  

Erdos had made his first significant contribution to number theory
  when he was 20, and discovered an elegant proof for the theorem which
  states that for each number greater than 1, there is always at least
  one prime number between it and its double. The Russian mathematician
  Chebyshev had proved this in the 19th century, but Erdos's proof was
  far neater. News of his success was passed around Hungarian
  mathematicians, accompanied by a rhyme: "Chebyshev said it, and I say
  it again/There is always a prime between n and 2n."

http://en.wikipedia.org/wiki/Bertrand%27s_postulate
A: Yes. If $n > 1$, then there is always at least one odd prime number $p$ satisfying $n < p < 2n$. In other words, $\pi(2n - 1) > \pi(n)$ for all $n > 1$, where $\pi(x)$ is the prime counting function. Look at Sloane's A060715 and you'll see the only 0 is for $n = 1$.
This is called Bertrand's postulate, but Chebyshev is the one who proved it first. Ramanujan and Erdős also came up with proofs. (By the way, just in case you're wondering, this does not prove the Goldbach conjecture.)
