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MathNet 2006 — Part 04 Question 133

Math Olympiad Vault · MathNet 2006 — Part 04

Question 133

There are $n \ge 3$ soldiers in the captain Petrenko's squad, and none two of them have the same height. The captain has drawn them all up into a single rank (not necessarily sorted by height). We call a "wave" any subsequence of soldiers in this rank (they are not supposed to stand next to each other) such that the first (leftmost) soldier in the wave is higher than the second soldier in it, but the second soldier in it is lower than the third one, who is in turn higher than the fourth one, and so on. (For example, if $n=9$, the soldiers are enumerated in ascending order by height, and the captain aligned them as 9-3-5-7-1-2-6-4-8, then one of the longest waves for this rank is 9-3-7-1-6-4-8. However, if the captain aligns them as 1-2-3-4-5-6-7-8-9, then each longest wave will consist of a single soldier, who can be anyone.) For every $n$, consider the number of possible ranks with the longest waves of even lengths and the number of possible ranks with the longest waves of odd lengths. Which of these numbers is bigger?