Given the sequence a1,a2,…,a100≥0 such that max{ai−1+ai,ai+ai+1}≥i for any 2≤i≤99, we need to find the minimum value of a1+a2+⋯+a100.
1. Analyzing the given condition:
max{ai−1+ai,ai+ai+1}≥i
This implies that either ai−1+ai≥i or ai+ai+1≥i (or both).
2. Constructing a sequence that satisfies the condition:
Let's consider a sequence where ai=2i for i=1,2,…,100.
3. Verifying the sequence:
For 2≤i≤99:
ai−1+ai=2i−1+2i=22i−1
ai+ai+1=2i+2i+1=22i+1
We need to check if:
max{22i−1,22i+1}≥i
Clearly, 22i+1≥i for all i, since:
22i+1=i+21≥i
4. Calculating the sum:
a1+a2+⋯+a100=21+22+⋯+2100=21+2+⋯+100
Using the formula for the sum of the first n natural numbers:
1+2+⋯+100=2100⋅101=5050
Therefore:
a1+a2+⋯+a100=25050=2525
5. Checking for a lower sum:
We need to check if there is a sequence that gives a lower sum while satisfying the condition. Consider the sequence where ai=0 for all i. This sequence does not satisfy the condition since max{ai−1+ai,ai+ai+1}=0 which is not ≥i.
Thus, the minimum sum of a1+a2+⋯+a100 that satisfies the given condition is 2525.
The final answer is 2525.