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Algebra Difficulty 6.9 National olympiad Prove it

32. Let {an}\left\{a_{n}\right\} be an infinite sequence of positive numbers. If for any i,aimi, a_{i} \leqslant m, and for any positive integers i,j(ij),aiaj1i+ji, j(i \neq j),\left|a_{i}-a_{j}\right| \geqslant \frac{1}{i+j}. Prove: m1m \geqslant 1.

Solution

32. For n4n \geqslant 4, let k1,k2,,knk_{1}, k_{2}, \cdots, k_{n} be a permutation of 1,2,,n1,2, \cdots, n, and satisfy 0<ak1<ak2<<aknm0 < a_{k_{1}} < a_{k_{2}} < \cdots < a_{k_{n}} \leqslant m. Thus, akiaki11ki+ki1(i=2,3,,n)a_{k_{i}} - a_{k_{i-1}} \geqslant \frac{1}{k_{i} + k_{i-1}} (i=2,3, \cdots, n). By the Cauchy-Schwarz inequality, maknak1=i=2n(akiaki1)i=2n1ki+ki1(n1)2i=2n(ki+ki1)=(n1)2n(n+1)k1kn(n1)2n2+n3=13n4n2+n3m \geqslant a_{k_{n}} - a_{k_{1}} = \sum_{i=2}^{n} \left(a_{k_{i}} - a_{k_{i-1}}\right) \geqslant \sum_{i=2}^{n} \frac{1}{k_{i} + k_{i-1}} \geqslant \frac{(n-1)^{2}}{\sum_{i=2}^{n} \left(k_{i} + k_{i-1}\right)} = \frac{(n-1)^{2}}{n(n+1) - k_{1} - k_{n}} \geqslant \frac{(n-1)^{2}}{n^{2} + n - 3} = 1 - \frac{3n - 4}{n^{2} + n - 3}. Note that this inequality holds for any n4n \geqslant 4, hence m1m \geqslant 1.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.