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jmc

number theory senior

Problem

A palindrome is a number that is the same when read forwards and backwards, such as . What is the smallest five-digit palindrome that is divisible by ?
Solution
For an integer to be divisible by then is divisible by

We start with the case where Then, Since we have a palindrome, we must have and meaning that We must have that and must be at least so then we can let and also be 1 and be zero. So the smallest such five-digit palindrome is

Then, we investigate the case where Then, and and so We see that we can let be 1 and be 9, then and we have the palindrome

Finally, Then, We check if has any solutions. We get so Then, we can see that there are no solutions for , since then we would have Since we already found we do not need to check for any greater than , so we see that our solution is
Final answer
10901