Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Prove it JBMO

Problem:
Find all positive integers nn such that 36n636^{n}-6 is a product of two or more consecutive positive integers.

Solution

Solution:
Answer: n=1n=1.

Among each four consecutive integers there is a multiple of 44. As 36n636^{n}-6 is not a multiple of 44, it must be the product of two or three consecutive positive integers.

Case I. If 36n6=x(x+1)36^{n}-6 = x(x+1) (all letters here and below denote positive integers), then 436n23=(2x+1)24 \cdot 36^{n} - 23 = (2x+1)^2, whence (26n+2x+1)(26n2x1)=23(2 \cdot 6^{n} + 2x + 1)(2 \cdot 6^{n} - 2x - 1) = 23. As 2323 is prime, this leads to 26n+2x+1=232 \cdot 6^{n} + 2x + 1 = 23, 26n2x1=12 \cdot 6^{n} - 2x - 1 = 1. Subtracting these yields 4x+2=224x + 2 = 22, x=5x = 5, n=1n = 1, which is a solution to the problem.

Case II. If 36n6=(y1)y(y+1)36^{n}-6 = (y-1)y(y+1), then
36n=y3y+6=(y3+8)(y+2)=(y+2)(y22y+3) 36^{n} = y^{3} - y + 6 = (y^{3} + 8) - (y + 2) = (y + 2)(y^{2} - 2y + 3)
Thus each of y+2y+2 and y22y+3y^{2} - 2y + 3 can have only 22 and 33 as prime factors, so the same is true for their GCD. This, combined with the identity y22y+3=(y+2)(y4)+11y^{2} - 2y + 3 = (y + 2)(y - 4) + 11 yields GCD(y+2,y22y+3)=1\operatorname{GCD}(y+2, y^{2} - 2y + 3) = 1. Now y+2<y22y+3y+2 < y^{2} - 2y + 3 and the latter number is odd, so y+2=4ny+2 = 4^{n}, y22y+3=9ny^{2} - 2y + 3 = 9^{n}. The former identity implies yy is even and now by the latter one 9n3(mod4)9^{n} \equiv 3 \pmod{4}, while in fact 9n1(mod4)9^{n} \equiv 1 \pmod{4} — a contradiction. So, in this case there is no such nn.

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