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Mongolian Mathematical Olympiad

Number theory Difficulty 5.7 AIME, harder Prove it Mongolia

Prove that if a1,a2,…,a2014a_1, a_2, \dots, a_{2014} are positive real numbers and there is no integer among them then there exists infinitely many nn such that (n;[a1n]+[a2n]+⋯+[a2014n])=1(n; [a_1n] + [a_2n] + \dots + [a_{2014n}]) = 1.

Solution

Proceeding by contradiction, suppose that there exists MM such that (n;[a1n]+[a2n]+⋯+[a2014n])≠1(n; [a_1n] + [a_2n] + \dots + [a_{2014n}]) \neq 1 for all n≥Mn \ge M. Consequently, for any prime pnp_n, (for all pn≥Mp_n \ge M) there exists xn∈Nx_n \in \mathbb{N} such that: [a1pn]+[a2pn]+⋯+[a2014pn]=pnxn[a_1p_n] + [a_2p_n] + \dots + [a_{2014p_n}] = p_nx_n. It is obvious that when n→∞n \to \infty the sequence xn=[a1pn]+[a2pn]+⋯+[a2014pn]pnx_n = \frac{[a_1p_n] + [a_2p_n] + \dots + [a_{2014p_n}]}{p_n} converges to a1+a2+⋯+a2014a_1 + a_2 + \dots + a_{2014} and because all terms of the sequence (xn)n≥N(x_n)_{n \ge N} are integers, there exists PP such that ∀n≥P:xn=a1+a2+⋯+a2014\forall n \ge P : x_n = a_1+a_2+\dots+a_{2014}. Since the latter is equivalent to ∀n≥P:{a1pn}+{a2pn}+⋯+{a2014pn}=0\forall n \ge P : \{a_1p_n\} + \{a_2p_n\} + \dots + \{a_{2014p_n}\} = 0, we conclude that ∀n≥P:aipn, i=1,2,…,2014\forall n \ge P : a_ip_n,\ i = 1, 2, \dots, 2014 are integers. It leads to that ai, i=1,2,…,2014a_i,\ i = 1, 2, \dots, 2014 are integers and but this is a contradiction.

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