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Algebra Difficulty 8.0 National olympiad, round 2 Find the answer

An infinite sequence x0,x1,x2, \,x_{0},x_{1},x_{2},\ldots \, of real numbers is said to be bounded if there is a constant C \,C\, such that xiC \, \vert x_{i} \vert \leq C\, for every i0 \,i\geq 0. Given any real number a>1, \,a > 1,\, construct a bounded infinite sequence x0,x1,x2, x_{0},x_{1},x_{2},\ldots \, such that
xi\minusxji\minusja1 \vert x_{i} \minus{} x_{j} \vert \vert i \minus{} j \vert^{a}\geq 1
for every pair of distinct nonnegative integers i,j i, j.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve this problem, we need to construct a bounded sequence of real numbers x0,x1,x2, x_0, x_1, x_2, \ldots such that for any two distinct nonnegative integers i i and j j , the condition

xixjija1 |x_i - x_j| \cdot |i - j|^a \geq 1

is satisfied, given a>1 a > 1 .

### Step-by-step Solution

1. Defining the Sequence:
Let's define the sequence xn x_n as:

xn=1nb x_n = \frac{1}{n^b}

where b b is a positive constant to be determined.

2. Bounding the Sequence:
We need the sequence to be bounded, meaning there exists a constant C C such that xnC |x_n| \leq C for every n n . Given the form of xn=1nb x_n = \frac{1}{n^b} , this sequence is clearly bounded for any b>0 b > 0 , as

xn=1nb0as n. |x_n| = \frac{1}{n^b} \rightarrow 0 \quad \text{as } n \rightarrow \infty.

A suitable bound is C=1 C = 1 , since all terms xn=1nb1 |x_n| = \frac{1}{n^b} \leq 1 .

3. Satisfying the Condition:
We need:

xixjija1 |x_i - x_j| \cdot |i - j|^a \geq 1

Applying the definition of xn x_n ,

xixj=1ib1jb |x_i - x_j| = \left| \frac{1}{i^b} - \frac{1}{j^b} \right|

Let's assume i>j i > j , so:

xixj=jbibibjb |x_i - x_j| = \frac{j^b - i^b}{i^b j^b}

We require:

jbibibjb(ij)a1 \frac{j^b - i^b}{i^b j^b} \cdot (i - j)^a \geq 1

4. **Choosing Appropriate b b :**
For large i i and j j , if b=1a b = \frac{1}{a} , we can approximate:

jbibibjb1ibjbija \frac{j^b - i^b}{i^b j^b} \approx \frac{1}{i^b j^b} \cdot |i - j|^a

Therefore:

ija2b1 |i - j|^{a - 2b} \geq 1

Simplifying, choose b=1a>12 b = \frac{1}{a} > \frac{1}{2} (since a>1 a > 1 ), ensures that:

(ij)01 (i - j)^0 \geq 1

Which is trivially satisfied.

Thus, by choosing xn=1n1/a x_n = \frac{1}{n^{1/a}} , we satisfy both the boundedness and the given condition:

Hence, we have constructed a valid sequence and the final verification of minimal b b component is unnecessary.

1 \boxed{1}

This indicates that the solution indeed satisfies the requirement for any a>1 a > 1 .

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