It seems that for any base $b\geq 2$, and for any number of digits $k\geq 2$, there is always some prime number that is $k$ digits long in base $b$.

For example, in base $10$, for $2\leq k\leq 10$ we have

$$\{11, 101, 1009, 10007, 100003, 1000003, 10000019, 100000007, 1000000007, 10000000019\}$$

and in base $2$ we have


are there any counterexamples to this or is there a proof that this is always the case?

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    $\begingroup$ Look up "Bertrand's postulate." $\endgroup$ – Mark Dickinson Jan 12 '15 at 18:09

Bertrand's postulate (proven later by Chebyshev) says that for any $n>1$, there exists some prime $p$ such that $n<p<2n$.

In particular, for base $b\ge2$ with $k\ge2$, this gives us that there exists a prime strictly between $b^{k-1}$ and $2b^{k-1}$, so we have that there exists a $k$-digit prime base $b$.


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