Find all polynomials of odd degree and with integer coefficients satisfying the following property: for each positive integer , there exists positive integers such that and is the -th power of a rational number for every pair of indices and with .
Solution
To solve this problem, we are tasked with finding all polynomials of odd degree with integer coefficients satisfying a specific condition. The condition states that for each positive integer , there exist positive integers such that the ratio lies strictly between and and is a -th power of a rational number for every pair of indices .
### Analysis
1. Polynomial Structure:
Since is of odd degree , we express it in the form:
The degree being odd ensures that the leading coefficient .
2. Condition on Ratios:
The condition that and is a -th power indicates certain divisibility and growth controls on . Rewriting this condition implies:
where is a reduced rational number and indicates that the ratio is indeed a -th power.
3. Implications on Form:
For the above to hold for arbitrary , particularly as grows, implies that the polynomial 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:
Here, are integers, with , , and 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 , expansion ensures integer coefficients since and are integer and relatively prime.
- Degree Check:
The degree of remains as desired.
- Condition Satisfaction:
For , the ratios naturally scale as -th powers of rational numbers, which also lie in the (1/2, 2) interval for sufficiently close choices of and .
With these considerations, we conclude that the polynomials satisfying all conditions are indeed of the form:
where are integers with , , and .
### Final Answer: