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Let $x_1, x_2,...,x_{2008}$ are numbers such that $|x_1|=999$ and for all $n=2,...,2008$
$$|x_n|=|x_{n-1}+1|$$ Find the smallest possible value of the sum
$$x_1+x_2+...+x_{2008}$$

My work:

Let $S=x_1+x_2+...+x_{2008}$.

If $x_1=-999, x_2=-998, ..., -1,0,-1,0$ then I think the answer $-500004$.

But I don't know how to prove that:

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Roman83
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  • Finding the smallest possible value of $|x_1+x_2+\cdots+x_{2008}|$ is also an interesting problem. (In fact, it's how I interpreted the problem on first reading.) – TonyK Jan 16 '19 at 12:58
  • The minimization problem can be understood as a multi-staging process in which the transition to a new staging, from $\sum_{k=1}^N x_k$ to $\sum_{k=1}^{N+1} x_k$ is constrained by the condition $ |x_{k+1}| = |x_k+1|$ so the procedure to solve it can be successfully handled with a Dynamic Programming algorithm. – Cesareo Jan 17 '19 at 11:04
  • On a side note, @Roman, let me ask you this: aren't you going to create an OEIS entry for the sequence from https://math.stackexchange.com/questions/2469058/a-n-is-the-smallest-positive-integer-number-such-that-sqrta-n-sqrta-n-1? IMHO, it more than deserves one. – Ivan Neretin Jan 17 '19 at 22:46
  • @IvanNeretin: No! I do not know how do it. But if you want you will can do it. – Roman83 Jan 18 '19 at 08:08
  • @Roman So I will. May I know your full name, at least? I don't want to claim it in my name, as the idea is yours. – Ivan Neretin Jan 18 '19 at 08:39
  • It is perfectly OK if you prefer to go anonymous, though. – Ivan Neretin Jan 18 '19 at 09:11
  • @Ivan Neretin: I think that the author of this problem is EdwardTurJ – Roman83 Jan 18 '19 at 11:18
  • @Roman Great. Now what's his full name? You see, they don't use nicknames there at OEIS. – Ivan Neretin Jan 18 '19 at 11:38
  • @IvanNeretin: His name is Edward Turkevich – Roman83 Jan 18 '19 at 13:18
  • @Roman Got it, thanks. – Ivan Neretin Jan 18 '19 at 13:19

1 Answers1

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Hint.

Calling

$$ S_1(x_1) = \{-m,m\} $$

and

$$ S_k(x_k) = \min_{x_k}\left(S_{k-1}(x_{k-1})+x_k\right)\ \ \mbox{s. t. }\ \ |x_k|=|x_{k-1}+1| $$

we have after $N$ steps the sought min as $\min S_N(x_N)$

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