12 can be made into a prime by changing the 2 to 3; likewise, 63 can be changed into a prime by changing the 6 to 7.

What is the smallest number that cannot changed into a prime by changing a single digit?

Are there infinitely many such numbers?

If the number was only one digit long, it could be changed to 2, 3, 5 or 7.

If it was two digits long, no matter what the tens digit was, we could change the units digit and produce a prime.

So... the number must be at least three digits long, but to solve this I need a larger table of primes, than the one in my head!

Posted by e.g. on 2004-04-29 08:25:59