Olympiad Maths Prep

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Number theory Difficulty 5.7 AIME, harder Prove it Austria

Find all pairs (a,b)(a, b) of non-negative integers such that
2017a=b632b+1. 2017^a = b^6 - 32b + 1.

Solution

Answer: The two solutions are (0,0)(0, 0) and (0,2)(0, 2).

Since 2017a2017^a is always odd, bb must be even, so b=2cb = 2c, cc integer. Therefore, 2017a=64(c6c)+12017^a = 64(c^6 - c) + 1 and thus 2017a1(mod64)2017^a \equiv 1 \pmod{64}. But we find 201733(mod64)2017 \equiv 33 \pmod{64} and 20172(1+32)2=1+232+3221(mod64)2017^2 \equiv (1+32)^2 = 1+2\cdot32+32^2 \equiv 1 \pmod{64}, so that the powers of 20172017 modulo 6464 alternate between 11 and 3333. Therefore, aa is even and 2017a2017^a is a perfect square. We denote the polynomial on the right-hand side of the given equation by r(b)=b632b+1r(b) = b^6 - 32b + 1 and show that it lies between two consecutive squares for b>4b > 4:

Let b>4b > 4. We have r(b)<b6=(b3)2r(b) < b^6 = (b^3)^2 for b>0b > 0. On the other hand, r(b)>(b31)2r(b) > (b^3 - 1)^2 because b632b+1>b62b3+1b>4b^6 - 32b + 1 > b^6 - 2b^3 + 1 \Leftrightarrow b > 4. Since the square 2017a2017^a is now between two consecutive squares, there are no solutions in this case.

Since bb is even, it remains to check b=4b = 4, b=2b = 2 and b=0b = 0.

For b=4b = 4, we regard the equation modulo 33 and get 112+1=01 \equiv 1 - 2 + 1 = 0, therefore, there is no solution in this case.

For b=2b = 2, we get 2017a=2626+12017^a = 2^6 - 2^6 + 1, so we get the solution (a,b)=(0,2)(a, b) = (0, 2).

For b=0b = 0, we get 2017a=12017^a = 1, so we get the solution (a,b)=(0,0)(a, b) = (0, 0).

Therefore, (0,0)(0, 0) and (0,2)(0, 2) are the only solutions.

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