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Number theory Difficulty 6.1 National Olympiad Prove it Romania

Find all four-digit numbers abcd\overline{abcd}, so that ab\overline{ab}, cb\overline{cb} and dd are prime numbers, and ab2+cb2+d2=2022\overline{ab}^2 + \overline{cb}^2 + d^2 = 2022.

Solution

If c=0c = 0, then b2+d2272b^2 + d^2 \le 2 \cdot 7^2, which means that ab\overline{ab} must be a prime number such that 1924ab2<20221924 \le \overline{ab}^2 < 2022, false. Therefore, c0c \ne 0. As ab\overline{ab} and cb\overline{cb} are primes, from the given equality we deduce that d=2d = 2. We obtain ab2+cb2=2018<472\overline{ab}^2 + \overline{cb}^2 = 2018 < 47^2, thus ab\overline{ab} and cb\overline{cb} are at most 43. Since bb is an odd digit, and the last digit of ab2+cb2\overline{ab}^2 + \overline{cb}^2 is 8, we deduce that b{3,7}b \in \{3, 7\}.

If b=7b = 7, then ab,cb{17,37}\overline{ab}, \overline{cb} \in \{17, 37\}, therefore ab2,cb2{289,1369}\overline{ab}^2, \overline{cb}^2 \in \{289, 1369\} and ab2+cb22018\overline{ab}^2 + \overline{cb}^2 \ne 2018.

If b=3b = 3, then ab,cb{13,23,43}\overline{ab}, \overline{cb} \in \{13, 23, 43\}, and we verify that only for (ab,cb){(13,43),(43,13)}(\overline{ab}, \overline{cb}) \in \{(13, 43), (43, 13)\} the given equality holds, therefore the solutions are abcd=1342\overline{abcd} = 1342 and abcd=4312\overline{abcd} = 4312.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.