For which positive integers does there exist an infinite arithmetic sequence of integers and an infinite geometric sequence of integers satisfying the following properties?
[list]
[*] is divisible by for all integers ;
[*] is not divisible by .
[/list]
[i]Holden Mui[/i]
Solution
Given:
1. An infinite arithmetic sequence of integers with the common difference .
2. An infinite geometric sequence of integers with the common ratio .
3. The condition that is divisible by for all .
4. The further condition that (the common difference) is not divisible by .
We need to determine for which positive integers these conditions hold.
### Analysis
Arithmetic and Geometric Sequences:
- For the arithmetic sequence, we have:
- For the geometric sequence, we have:
Difference Condition:
The condition implies:
Rewriting, this gives:
### Divisibility Argument
The first condition holds for all if and only if and
For the second sequence property , this indicates .
Condition Conclusions:
- If is squarefree, then every divisor of is prime, such that (for any prime divisor ) forces , contradicting the non-zero difference condition since otherwise, all differences would need to be zero modulo .
- If is not squarefree, i.e., contains a repeated prime factor , we can choose but , allowing for the divisibility of differences to meet all conditions without .
### Conclusion
Thus, must be not squarefree for these sequences with the conditions given to exist.