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Number theory Difficulty 6.6 National olympiad Prove it

36 Let pp be a prime, aa and nn be positive integers, and 2p+3p=an2^{p}+3^{p}=a^{n}. Prove: n=1n=1.

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Solution

36. When p=2p=2, an=13a^{n}=13, we know a=13,n=1a=13, n=1. When p>2p>2, since pp is a prime, pp is an odd number, at this time
2p+3p=(2+3)(2p12p2×3+2×3p2+3p1),2^{p}+3^{p}=(2+3)\left(2^{p-1}-2^{p-2} \times 3+\cdots-2 \times 3^{p-2}+3^{p-1}\right),

thus 5an5 \mid a^{n}, which means 5a5 \mid a. If n>1n>1, then 52an5^{2} \mid a^{n}, in this case, we should have
2p12p2×3+2×3p2+3p10(mod5)2^{p-1}-2^{p-2} \times 3+\cdots-2 \times 3^{p-2}+3^{p-1} \equiv 0(\bmod 5)

Using 32(mod5)3 \equiv-2(\bmod 5), pp is an odd number, and the above equation, we know
2p12p2×3+2×3p2+3p12p1+2p1++2p1p2p1=p2p10(mod5)\begin{aligned} & 2^{p-1}-2^{p-2} \times 3+\cdots-2 \times 3^{p-2}+3^{p-1} \\ \equiv & \underbrace{2^{p-1}+2^{p-1}+\cdots+2^{p-1}}_{p \uparrow 2^{p-1}} \\ = & p \cdot 2^{p-1} \equiv 0(\bmod 5) \end{aligned}

Therefore, 5p5 \mid p, and since pp is a prime, p=5p=5, which leads to an=25+35=275=52×11,na^{n}=2^{5}+3^{5}=275=5^{2} \times 11, n can only be 1, a contradiction.

Thus, n=1n=1.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.