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Number theory Difficulty 6.3 National olympiad Prove it Belarus

Do there exist three two-digit prime numbers such that all six digits in their decimal representation are distinct and if one chooses any two of the numbers and swaps their unit digits, then two numbers obtained will be prime, too?

Solution

Answer: there are no such numbers.
Suppose, contrary to our claim, that there are such numbers: ab\overline{ab}, xy\overline{xy} and pq\overline{pq}. The problem condition is equivalent to the following statement: any two-digit number with tens digit aa, xx or pp, and with unit digit bb, yy or qq is a prime number. So b,y,q{1,3,7,9}b, y, q \in \{1, 3, 7, 9\}, hence, at least one of the digits 3 and 9 belongs to {b,y,q}\{b, y, q\}. Therefore, none of the digits a,x,pa, x, p is divisible by 3, so a,x,pM={1,2,4,5,7,8}a, x, p \in M = \{1, 2, 4, 5, 7, 8\}. Thus, we can consider two-digit numbers with tens digits from the set MM only. Among numbers 11, 21, 41, 51, 71, 81 there are exactly two prime numbers with distinct tens and unit digits: 41 and 71. Therefore, none of the digits b,y,qb, y, q is equal to 1. It follows that {b,y,q}={3,7,9}\{b, y, q\} = \{3, 7, 9\}, which is impossible. Indeed, exactly two numbers 17 and 47 are prime among two-digit numbers 17, 27, 47, 57, 77, 87, while there should be at least three prime numbers among these numbers.

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