Number theoryDifficulty 5.9AIME, harderProve itUnited States
Problem: Does there exist an irrational number α>1 such that ⌊αn⌋≡0(mod2017) for all integers n≥1?
Solution
Solution: Answer: Yes Let α>1 and 0<β<1 be the roots of x2−4035x+2017. Then note that ⌊αn⌋=αn+βn−1. Let xn=αn+βn for all nonnegative integers n. It's easy to verify that xn=4035xn−1−2017xn−2≡xn−1(mod2017) so since x1=4035≡1(mod2017) we have that xn≡1(mod2017) for all n. Thus α satisfies the problem.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement reproduced verbatim; metadata (topic, difficulty) added by this project.