Maths Olympiad Prep

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Number theory Difficulty 4.1 AIME Find the answer United States

Problem:
The number 27,000,00127,000,001 has exactly four prime factors. Find their sum.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
First, we factor
27x6+1=(3x2)3+1=(3x2+1)(9x43x2+1)=(3x2+1)((9x4+6x2+1)9x2)=(3x2+1)((3x2+1)2(3x)2)=(3x2+1)(3x2+3x+1)(3x23x+1) \begin{aligned} 27x^{6} + 1 &= (3x^{2})^{3} + 1 \\ &= (3x^{2} + 1)(9x^{4} - 3x^{2} + 1) \\ &= (3x^{2} + 1)((9x^{4} + 6x^{2} + 1) - 9x^{2}) \\ &= (3x^{2} + 1)((3x^{2} + 1)^{2} - (3x)^{2}) \\ &= (3x^{2} + 1)(3x^{2} + 3x + 1)(3x^{2} - 3x + 1) \end{aligned}
Letting x=10x = 10, we get that 27,000,001=30133127127,000,001 = 301 \cdot 331 \cdot 271. A quick check shows that 301=743301 = 7 \cdot 43, so that 27,000,001=74327133127,000,001 = 7 \cdot 43 \cdot 271 \cdot 331. Each factor here is prime, and their sum is 652652.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.