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Algebra Difficulty 8.9 Shortlist Find the answer

Find all polynomials P(x)P(x) of odd degree dd and with integer coefficients satisfying the following property: for each positive integer nn, there exists nn positive integers x1,x2,,xnx_1, x_2, \ldots, x_n such that 12<P(xi)P(xj)<2\frac12 < \frac{P(x_i)}{P(x_j)} < 2 and P(xi)P(xj)\frac{P(x_i)}{P(x_j)} is the dd-th power of a rational number for every pair of indices ii and jj with 1i,jn1 \leq i, j \leq n.

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

Solution

To solve this problem, we are tasked with finding all polynomials P(x) P(x) of odd degree d d with integer coefficients satisfying a specific condition. The condition states that for each positive integer n n , there exist n n positive integers x1,x2,,xn x_1, x_2, \ldots, x_n such that the ratio P(xi)P(xj) \frac{P(x_i)}{P(x_j)} lies strictly between 12\frac{1}{2} and 22 and is a dd-th power of a rational number for every pair of indices i,j i, j .

### Analysis

1. Polynomial Structure:

Since P(x) P(x) is of odd degree d d , we express it in the form:
P(x)=adxd+ad1xd1++a1x+a0 P(x) = a_d x^d + a_{d-1} x^{d-1} + \cdots + a_1 x + a_0

The degree d d being odd ensures that the leading coefficient ad0 a_d \neq 0 .

2. Condition on Ratios:

The condition that 12<P(xi)P(xj)<2\frac{1}{2} < \frac{P(x_i)}{P(x_j)} < 2 and P(xi)P(xj)\frac{P(x_i)}{P(x_j)} is a dd-th power indicates certain divisibility and growth controls on P(x) P(x) . Rewriting this condition implies:

P(xi)=(pq)dP(xj) P(x_i) = \left(\frac{p}{q}\right)^d P(x_j)

where (pq)\left(\frac{p}{q}\right) is a reduced rational number and (p/q)d(p/q)^d indicates that the ratio is indeed a dd-th power.

3. Implications on Form:

For the above to hold for arbitrary n n , particularly as n n grows, implies that the polynomial P(x) P(x) must retain a consistent ratio property. This strongly suggests a form based on scaled and shifted integer variables.

4. Determining the Polynomial:

A suitable candidate satisfying these conditions is:
P(x)=a(rx+s)d P(x) = a(rx + s)^d

Here, a,r,s a, r, s are integers, with a0 a \neq 0 , r1 r \geq 1 , and (r,s)=1 (r, s) = 1 ensuring that the transformation and scaling do not introduce any non-integer terms or additional roots that disrupt the integer coefficient condition.

### Validation:

- Integer Coefficients:
By the form (rx+s)d (rx+s)^d, expansion ensures integer coefficients since rr and ss are integer and relatively prime.

- Degree Check:
The degree of P(x) P(x) remains dd as desired.

- Condition Satisfaction:
For P(xi)P(xj)=(rxi+srxj+s)d \frac{P(x_i)}{P(x_j)} = \left(\frac{rx_i+s}{rx_j+s}\right)^d , the ratios naturally scale as dd-th powers of rational numbers, which also lie in the (1/2, 2) interval for sufficiently close choices of xi x_i and xj x_j .

With these considerations, we conclude that the polynomials satisfying all conditions are indeed of the form:
P(x)=a(rx+s)d P(x) = a(rx + s)^d
where a,r,s a, r, s are integers with a0 a \neq 0 , r1 r \geq 1 , and (r,s)=1 (r, s) = 1 .

### Final Answer:
P(x)=a(rx+s)d where a,r,s are integers with a0,r1 and (r,s)=1. \boxed{P(x) = a(rx + s)^d \text{ where } a, r, s \text{ are integers with } a \neq 0, r \geq 1 \text{ and } (r, s) = 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.