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Number theory Difficulty 5.9 AIME, harder Prove it Mongolia

Show that the equation x4+x3+x2+x=y6+61x^4 + x^3 + x^2 + x = y^6 + 61 has no integer solutions.

Solution

Suppose there exist integers xx and yy such that x4+x3+x2+x=y6+61x^4 + x^3 + x^2 + x = y^6 + 61.

Let us consider the equation modulo 77.

First, note that y60y^6 \equiv 0 or 1(mod7)1 \pmod{7} for any integer yy, since by Fermat's Little Theorem, y61y^6 \equiv 1 if yy is not divisible by 77, and 00 otherwise.

So y60y^6 \equiv 0 or 1(mod7)1 \pmod{7}.

Now, 615(mod7)61 \equiv 5 \pmod{7}, so y6+615y^6 + 61 \equiv 5 or 6(mod7)6 \pmod{7}.

Therefore, x4+x3+x2+x5x^4 + x^3 + x^2 + x \equiv 5 or 6(mod7)6 \pmod{7}.

Let us compute x4+x3+x2+x(mod7)x^4 + x^3 + x^2 + x \pmod{7} for all x=0,1,2,3,4,5,6x = 0, 1, 2, 3, 4, 5, 6:

- x=0x = 0: 04+03+02+0=00^4 + 0^3 + 0^2 + 0 = 0
- x=1x = 1: 1+1+1+1=41 + 1 + 1 + 1 = 4
- x=2x = 2: 16+8+4+2=302(mod7)16 + 8 + 4 + 2 = 30 \equiv 2 \pmod{7}
- x=3x = 3: 81+27+9+3=1201(mod7)81 + 27 + 9 + 3 = 120 \equiv 1 \pmod{7}
- x=4x = 4: 256+64+16+4=3404(mod7)256 + 64 + 16 + 4 = 340 \equiv 4 \pmod{7}
- x=5x = 5: 625+125+25+5=7803(mod7)625 + 125 + 25 + 5 = 780 \equiv 3 \pmod{7}
- x=6x = 6: 1296+216+36+6=15541(mod7)1296 + 216 + 36 + 6 = 1554 \equiv 1 \pmod{7}

So the possible residues are 0,1,2,3,40, 1, 2, 3, 4 modulo 77.

But y6+615y^6 + 61 \equiv 5 or 6(mod7)6 \pmod{7}, which are not among the possible residues for x4+x3+x2+xx^4 + x^3 + x^2 + x.

Therefore, there are no integer solutions to the equation.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.