Find all the positive perfect cubes that are not divisible by such that the number obtained by erasing the last three digits is also a perfect cube.
Solution
To solve the given problem, we need to find all positive perfect cubes that are not divisible by and have the property that when the last three digits are erased, the resulting number is also a perfect cube.
1. Understanding the Cube Condition: Let be a perfect cube such that . This means the last three digits of are zero, making a multiple of . However, the problem requires that should not be divisible by 10; that is, neither should end in zero nor should it be divisible by 5.
2. Formulation: Let , where represents the remaining last three digits. Given that erasing the last three digits of results in a perfect cube, we have for some integer .
3. Range of Interest: We need to check for values since we are initially interested in cubes where erasing might yield smaller cubes.
4. Computations:
- For , we have , since . When the last three digits are erased, the number is , which is .
- For , we have , since . When the last three digits are erased, the number is , which is .
5. Verification:
- Check if these cubes satisfy the divisibility condition. Both and are not divisible by .
- Check if the condition upon erasing the last three digits is a cube. We already verified that in each case the number becomes , which is indeed a cube since .
Therefore, the perfect cubes that satisfy the conditions are: