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Number theory Difficulty 6.7 National olympiad Find the answer

Are there integers aa and bb such that a5b+3a^5b+3 and ab5+3ab^5+3 are both perfect cubes of integers?

A number or a short expression. Spacing and $ signs are ignored.

Solution

No, such integers do not exist. This shall be proven by contradiction, by showing that if a5b+3a^5b+3 is a perfect cube then ab5+3ab^5+3 cannot be.
Remark that perfect cubes are always congruent to 00 , 11 , or 1-1 modulo 99 . Therefore, if a5b+30,1, or1(mod9)a^5b+3\equiv 0,1,\text{ or} -1\pmod{9} , then a5b5,6, or 7(mod9)a^5b\equiv 5,6,\text{ or }7\pmod{9} .
If a5b6(mod9)a^5b\equiv 6\pmod 9 , then note that 3b3|b . (This is because if 3a3|a then a5b0(mod9)a^5b\equiv 0\pmod 9 .) Therefore ab50(mod9)ab^5\equiv 0\pmod 9 and ab5+33(mod9)ab^5+3\equiv 3\pmod 9 , contradiction.
Otherwise, either a5b5(mod9)a^5b\equiv 5\pmod 9 or a5b7(mod9)a^5b\equiv 7\pmod 9 . Note that since a6b6a^6b^6 is a perfect sixth power, and since neither aa nor bb contains a factor of 33 , a6b61(mod9)a^6b^6\equiv 1\pmod 9 . If a5b5(mod9)a^5b\equiv 5\pmod 9 , then a6b6(a5b)(ab5)5ab51(mod9)    ab52(mod9).a^6b^6\equiv (a^5b)(ab^5)\equiv 5ab^5\equiv 1\pmod 9\implies ab^5\equiv 2\pmod 9. Similarly, if a5b7(mod9)a^5b\equiv 7\pmod 9 , then a6b6(a5b)(ab5)7ab51(mod9)    ab54(mod9).a^6b^6\equiv (a^5b)(ab^5)\equiv 7ab^5\equiv 1\pmod 9\implies ab^5\equiv 4\pmod 9. Therefore ab5+35,7(mod9)ab^5+3\equiv 5,7\pmod 9 , contradiction.
Therefore no such integers exist.
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