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Brazil

Brazil number theory

Problem

A natural number is a palindrome when one obtains the same number when writing its digits in reverse order. For example, , and are palindromes. Determine all pairs of positive integers such that is a palindrome.
Solution
Note that has exactly digits, since . If , then considering the tenth leftmost digit, there would be a carry, thus the number consisting of the first nine digits of is bigger than the number formed by the last nine digits of , in reversed order. Then if and are bigger than the number is not a palindrome.

If one of the numbers , does not exceed then there won't be a carry and thus the number is palindrome.
Final answer
All pairs with min(m, n) ≤ 9; equivalently, at least one of m or n is at most nine.

Techniques

OtherIntegers