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?
The following infinite sequence will work:
200, 23300, 46400, ...
where the difference is 23100
The sum of the digits of each of these is 1 less than a multiple of 3 so that changing the last digit to a 1 or a 7 will make the number a multiple of 3.
203 is a multiple of 7 as is the difference, 23100, so whenever the last digit of any of these is changed to 3, the number is a multiple of 7.
209 is a multiple of 11 and so is 23100, so in all the cases where the last digit is changed to 9 the result is a multiple of 11.
The even last digits are again multiples of 2 and a last digit of 5 results in a multiple of 5. Changing any digit but the last leaves a multiple of 10.
There are of course numbers that fit the pattern that are not on this sequence, but this proves the infinite number of such numbers.
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Posted by Charlie
on 2004-04-29 11:25:55 |