Maths Olympiad Prep

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, 2017

Number theory Difficulty 5.9 AIME, harder Prove it United States

Problem:
Does there exist an irrational number α>1\alpha>1 such that
αn0(mod2017) \left\lfloor\alpha^{n}\right\rfloor \equiv 0 \quad(\bmod 2017)
for all integers n1n \geq 1?

Solution

Solution:
Answer: Yes
Let α>1\alpha>1 and 0<β<10<\beta<1 be the roots of x24035x+2017x^{2}-4035 x+2017. Then note that αn=αn+βn1\left\lfloor\alpha^{n}\right\rfloor=\alpha^{n}+\beta^{n}-1. Let xn=αn+βnx_{n}=\alpha^{n}+\beta^{n} for all nonnegative integers nn. It's easy to verify that xn=4035xn12017xn2xn1x_{n}=4035 x_{n-1}-2017 x_{n-2} \equiv x_{n-1} (mod2017)(\bmod 2017) so since x1=40351(mod2017)x_{1}=4035 \equiv 1(\bmod 2017) we have that xn1(mod2017)x_{n} \equiv 1(\bmod 2017) for all nn. Thus α\alpha satisfies the problem.

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