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?

(In reply to A simple solution for the 2nd part by e.g.)

So wouldn't all multiples of 11! work?  It would end with zero, and since it has 2-9 and 11 as factors, adding 2-9 or 11 (making it end with 1-9) would still be composite.

Posted by Ryan on 2004-04-30 16:40:21