Determine the least positive integer such that can be written as a sum of palindromic primes.
[A palindromic number is a positive integer—like —whose decimal representation remains the same when its digits are reversed.]
Solution
Since is neither a prime nor a palindromic number, . Suppose . Then there are two palindromic primes such that . Since isn't prime, . Say . Then . But palindromic numbers ending in are even, and so can't be prime. Hence, . Also, every number of the form , where , is divisible by . Hence, there are no palindromic primes of this form, and so . This means that , which is absurd. Hence, .
If , then there are three palindromic primes such that . If one of is even, say , then and the argument just given shows that this can't happen. But the sum of three odd numbers is always odd, hence never equal to . Thus, .
In fact, , since
[It can be checked that , constitute the complete list of palindromic primes less than .]
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