AlgebraDifficulty 7.8National olympiad, round 2Find the answer
Find all integers n≥2 for which there exist the real numbers ak,1≤k≤n, which are satisfying the following conditions: k=1∑nak=0,k=1∑nak2=1 and n⋅(k=1∑nak3)=2(bn−1), where b=1≤k≤nmax{ak}.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Let us consider the given problem and analyze the conditions set by the equations:
1. ∑k=1nak=0, 2. ∑k=1nak2=1, 3. n⋅(∑k=1nak3)=2(bn−1), where b=max1≤k≤n{ak}.
The task is to find all integers n≥2 for which these real numbers ak exist.
### Step 1: Analyze Sum Constraints
Given ∑k=1nak=0, the sum of the numbers ak is zero. This indicates a balance between positive and negative terms among ak.
### Step 2: Analyze Maximum Constraint
Let b=max1≤k≤n{ak}. From the constraint n⋅(∑k=1nak3)=2(bn−1), we must have: k=1∑nak3=n2bn−2=2b−n2.
### Step 3: Consider Equality Conditions
We now explore whether the equality conditions can hold for different values of n:
- For n=2: Consider a1=b and a2=−b. Then: k=1∑2ak=b−b=0, k=1∑2ak2=b2+b2=2b2=1⇒b2=21, k=1∑2ak3=b3−b3=0, 2⋅(0)=2(b2−1)⇒b2=1.
This gives b=21, which satisfies all conditions for n=2.
### Step 4: Generalize for Even n>2
Considering n being even: - Let ak=n1 for k in the first half and ak=−n1 for k in the second half. - Then: k=1∑nak=0, k=1∑nak2=nn=1, k=1∑nak3=(n)3n/2−(n)3n/2=0, n⋅0=2(n1n−1)=0.
Hence, the conditions are satisfied for all even n.
### Conclusion:
The integers n for which the conditions are met are all even integersn≥2. Thus, the final answer is: even integers n≥2
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