# Need a 12-Digit Prime?

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You may have seen the joke “Enter any 12-digit prime number to continue.” I’ve seen it floating around as the punchline in several contexts.

So what do you do if you need a 12-digit prime? Here’s how to find the smallest one using Python.

>>> from sympy import nextprime >>> nextprime(10**11) 100000000003L

The function `nextprime`

gives the smallest prime larger than its argument. (Note that the smallest 12-digit number is 10^{11}, not 10^{12}. Great opportunity for an off-by-one mistake.)

Optionally you can provide an addition argument to `nextprime`

to get primes further down the list. For example, this gives the second prime larger than 10^{11}.

>>> nextprime(10**11, 2) 100000000019L

What if you wanted the largest 12-digit prime rather than the smallest?

>>> from sympy import prevprime >>> prevprime(10**12) 999999999989L

Finally, suppose you want to know how many 12-digit primes there are. SymPy has a function`primepi`

that returns the number of primes less than its argument. Unfortunately, it fails for large arguments. It works for arguments as big as `2**27`

but throws a memory error for `2**28`

.

The number of primes less than *n* is approximately *n* / log *n, *so there are about 32 billion primes between 10^{11} and 10^{12}. According to Wolfram Alpha, the exact number of 12-digit primes is 33,489,857,205. So if you try 12-digit numbers at random, your chances are about 1 in 30 of getting a prime. If you’re clever enough to just pick odd numbers, your chances go up to 1 in 15.

Published at DZone with permission of John Cook, DZone MVB. See the original article here.

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