### Fly_37's blog

By Fly_37, history, 22 months ago,

Be careful not to provide your password there! This is a scam. After clicking on register page you are NOT getting redirected to codeforces!

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By Fly_37, history, 23 months ago,

Hello!

Some time ago I created a problem for local programming competition. Unfortunately it turned out that I had incomplete proof of one lemma, that I can not show even to this day.

Lemma: Given an increasing array of $N$ arbitrary large numbers we define its cost as sum of lengths of all non-trivial, maximal arithmetic progressions starting at the first element. The cost of any array is $\mathcal{O}(N\log{N})$.

For example for array $[0, 2, 3, 4, 6, 8, 9]$ — the total cost is $|[0, 2, 4, 6, 8]| + |[0, 3, 6, 9]| + |[0, 4, 8]| + |[0, 6]| + |[0, 8]| + |[0, 9]| = 5 + 4 + 3 + 2 + 2 + 2= 18$.

It is easy to see, that if we simply take $N$ consecutive natural numbers we get $\mathcal{O}(N\log{N})$ cost, but I was not able to prove that this is the worst case scenario.

Best complexity I can show is $\mathcal{o}(N^2)$, but still far from the goal...

Can anyone show if the lemma is true or false?

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