1993 USAMO Problems/Problem 5
Let be a sequence of positive real numbers satisfying for . (Such a sequence is said to be log concave.) Show that for each ,
Notice that because we may subtract from both sides of the inequality and observe that it is sufficient to prove that or
Fortunately, this is an easy inequality. Indeed, from AM-GM applied on each group of terms we have and so it suffices to prove or, after taking both sides to the power, simplifying, and taking the -th root of both sides, to prove This easily follows from the Fact that for . Indeed, we are given that Multiply all inequalities together and cancel to give . Similarly, by multiplying all inequalities except the first and the last, we deduce that , and a simple induction argument proves the verity of the Fact for , and so by the Commutative Property the Fact is true for all , as desired. Now multiply each inequality of the Fact for to give the desired result.
Note: The Fact can be generalized into a Lemma: whenever and . The proof is similar to that of the Fact and is left as an exercise to the reader.
--User suli, April 15, 2015.
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