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Algebra Difficulty 7.8 National olympiad, round 2 Find the answer

Find all integers n2n\geq 2 for which there exist the real numbers ak,1kna_k, 1\leq k \leq n, which are satisfying the following conditions:
k=1nak=0,k=1nak2=1 and n(k=1nak3)=2(bn1), where b=max1kn{ak}.\sum_{k=1}^n a_k=0, \sum_{k=1}^n a_k^2=1 \text{ and } \sqrt{n}\cdot \Bigr(\sum_{k=1}^n a_k^3\Bigr)=2(b\sqrt{n}-1), \text{ where } b=\max_{1\leq k\leq n} \{a_k\}.

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\sum_{k=1}^n a_k = 0,
2. k=1nak2=1\sum_{k=1}^n a_k^2 = 1,
3. n(k=1nak3)=2(bn1)\sqrt{n} \cdot \left(\sum_{k=1}^n a_k^3\right) = 2(b\sqrt{n} - 1), where b=max1kn{ak}b = \max_{1 \leq k \leq n} \{a_k\}.

The task is to find all integers n2n \geq 2 for which these real numbers aka_k exist.

### Step 1: Analyze Sum Constraints

Given k=1nak=0\sum_{k=1}^n a_k = 0, the sum of the numbers aka_k is zero. This indicates a balance between positive and negative terms among aka_k.

### Step 2: Analyze Maximum Constraint

Let b=max1kn{ak}b = \max_{1 \leq k \leq n} \{a_k\}. From the constraint n(k=1nak3)=2(bn1), \sqrt{n} \cdot \left(\sum_{k=1}^n a_k^3\right) = 2(b\sqrt{n} - 1), we must have:
k=1nak3=2bn2n=2b2n. \sum_{k=1}^n a_k^3 = \frac{2b\sqrt{n} - 2}{\sqrt{n}} = 2b - \frac{2}{\sqrt{n}}.

### Step 3: Consider Equality Conditions

We now explore whether the equality conditions can hold for different values of nn:

- For n=2n = 2: Consider a1=ba_1 = b and a2=ba_2 = -b. Then:
k=12ak=bb=0, \sum_{k=1}^2 a_k = b - b = 0,
k=12ak2=b2+b2=2b2=1b2=12, \sum_{k=1}^2 a_k^2 = b^2 + b^2 = 2b^2 = 1 \Rightarrow b^2 = \frac{1}{2},
k=12ak3=b3b3=0, \sum_{k=1}^2 a_k^3 = b^3 - b^3 = 0,
2(0)=2(b21)b2=1. \sqrt{2} \cdot (0) = 2(b \sqrt{2} - 1) \Rightarrow b \sqrt{2} = 1.

This gives b=12b = \frac{1}{\sqrt{2}}, which satisfies all conditions for n=2n = 2.

### Step 4: Generalize for Even n>2n > 2

Considering nn being even:
- Let ak=1na_k = \frac{1}{\sqrt{n}} for kk in the first half and ak=1na_k = -\frac{1}{\sqrt{n}} for kk in the second half.
- Then:
k=1nak=0, \sum_{k=1}^n a_k = 0,
k=1nak2=nn=1, \sum_{k=1}^n a_k^2 = \frac{n}{n} = 1,
k=1nak3=n/2(n)3n/2(n)3=0, \sum_{k=1}^n a_k^3 = \frac{n/2}{(\sqrt{n})^3} - \frac{n/2}{(\sqrt{n})^3} = 0,
n0=2(1nn1)=0. \sqrt{n} \cdot 0 = 2\left(\frac{1}{\sqrt{n}} \sqrt{n} - 1\right) = 0.

Hence, the conditions are satisfied for all even nn.

### Conclusion:

The integers nn for which the conditions are met are all even integers n2n \geq 2. Thus, the final answer is:
even integers n2 \boxed{\text{even integers } n \geq 2}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.