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Number theory Difficulty 4.8 AIME Prove it Serbia

Problem:

Solve the following equation in the set of nonnegative integers
(22015+1)x+22015=2y+1. \left(2^{2015}+1\right)^{x}+2^{2015}=2^{y}+1.

Solutions — 2

Solution 1

Solution:

For x1x \leqslant 1 the only solutions are (0,2015)(0,2015) and (1,2016)(1,2016).

Let x>1x>1. Since 22015+12^{2015}+1 is divisible by 33, we have
(22015+1)x+22015220155(mod9), \left(2^{2015}+1\right)^{x}+2^{2015} \equiv 2^{2015} \equiv 5 \pmod{9},
hence 2y4(mod9)2^{y} \equiv 4 \pmod{9}, from which we obtain y=6k+2y=6k+2 for some kNk \in \mathbb{N}.

Now modulo 1313 we have 2y+1=(26)k22±42^{y}+1=\left(2^{6}\right)^{k} \cdot 2^{2} \equiv \pm 4 and 220157(mod13)2^{2015} \equiv 7 \pmod{13}, so we obtain 8x+7±4(mod13)8^{x}+7 \equiv \pm 4 \pmod{13}. However, 8x8^{x} gives one of the residues 1,5,8,121,5,8,12 modulo 1313, so the last congruence is impossible.

Solution 2

Solution:

For x>1x>1 we have
2y=(22015+1)x+220151=(x+1)22015+i=2x(xi)22015i(x+1)22015(mod22019), 2^{y}=\left(2^{2015}+1\right)^{x}+2^{2015}-1=(x+1)2^{2015}+\sum_{i=2}^{x}\binom{x}{i}2^{2015i} \equiv (x+1)2^{2015} \pmod{2^{2019}},
so since y>2019y>2019 it follows that 16x+116 \mid x+1.

Now consider the equation modulo 1717. Since 220159(mod17)2^{2015} \equiv 9 \pmod{17}, we obtain 2y10x+81015+83(mod17)2^{y} \equiv 10^{x}+8 \equiv 10^{15}+8 \equiv 3 \pmod{17}, which is impossible.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from sr; metadata (topic, difficulty) added by this project.