... n`ao¯d´o. Dov.ây, w(n) ≤ j − 1 v`a 2 w(n)4√n≤ 2 j−14√p1p 2 · · · pj−1.V`ı v.âyk =max0, 2 14√ 2. 3, 2 24√ 2. 3.5, · · ·= 2 54√ 2. 3.5.7.11.13= 5.Bây gi`o,ta ... Ðê` thi olympic to´an IrelandTheo b´˘at¯d,˘ang th´u,c Jensen’s ta c´o:f (a) + f (b) 2 ≥ f (a + b 2 ) ⇒1a+1b 2 ≥ 2 a + b.f (b) + f (c) 2 ≥ f (b + c 2 ) ⇒1b+1c 2 ≥ 2 b ... n´o. Khi ch´ung ta c´o 2 7(n+ 1) 2 ¯d,ınh c´o m`au n`ay, ch´ung ta c´ol(n) ≤ 2 7(n+ 1) 2 ≤ 2 7(n + 1) 2 do v.âyl(n)n 2 ≤ 2 7n + 7n 2 .Khi n →∝, th`ı vê´ph,ai...