# What's the smallest number that we don't know if it's prime or composite? [duplicate]

What's the biggest $n$ such that for all $1<x<n$, we know for sure if $x$ is prime?

The smallest primes are easy to find, and the biggest ones we haven't found yet. At the top, we have Mersenne primes, but not all primes are Mersenne primes. There's an unclear boundry, up to which we know some of the primes, but probably not all.

## marked as duplicate by hardmath, SchrodingersCat, Community♦Jan 21 '16 at 19:06

• Who are "we"? Suppose all numbers $2$ through $k-2$ have been checked by someone in the world at some point, $k$ has never been checked, and last year someone somewhere privately checked $k-1$ but never told anyone else the result. Does that make $k$ the largest $n$? If not, what is the standard of when "we" know whether a number is prime? – David K Jan 21 '16 at 13:13