A -*sequence* is a sequence of numbers each equal to either or . Determine the largest so that, for any -sequence, there exists an integer and indices so that for all , and
Solution
To solve the given problem, we first need to understand the requirements for a -sequence. We are looking for the largest integer such that, for any sequence of numbers where each is either or , there exists a subsequence satisfying certain conditions.
### Problem Breakdown:
- We need to identify a subsequence with:
- Indices such that .
- The difference between consecutive indices is .
- The absolute value of the sum of the subsequence elements satisfies .
### Solution Approach:
The essence of the problem is to ensure that there always exists a subsequence where the sum is as large as possible given the constraints on indices.
Key Idea:
- If we take any two consecutive elements in the sequence, denoted as and , the sum can be (if ), (if both are ), or (if both are ).
- Hence, checking groups of 3 consecutive elements, and , we can form subsequences with sums of absolute value 1, 2, or 3.
- The strategy is to maximize the deviation by optimally grouping these sequences.
Construction:
1. Analyze small groups of consecutive terms such as , and decide whether to take two or more elements to maximize the absolute sum.
2. Since , there are effectively full groups of 3 elements we can evaluate, plus one extra element.
3. For each group of three elements , the maximum absolute sum we can always achieve is 1, capturing the nature of any sequence distribution.
Conclusion:
By consistently leveraging groups of up to three elements, thereby utilizing every potential subsequence opportunity with , the minimum can be achieved across groups:
- For each of the 673 groups of three consecutive numbers, we can guarantee a sum of magnitude 1, resulting in as the worst-case minimum.
Thus, the largest we can guarantee for any -sequence under the given conditions is: