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jmc

number theory intermediate

Problem

How many numbers from to are not perfect squares or perfect cubes?
Solution
The largest perfect square less than is . Therefore, there are perfect squares between and .

The largest perfect cube less than is . Therefore, there are perfect cubes between and .

However, there are numbers between and that are both perfect squares and perfect cubes. For a number to be both a perfect square and perfect cube, it must be a 6th power. The largest sixth power less than is . Therefore, there are sixth powers between and . Those two numbers are counted twice, so we have to subtract from the number of numbers that are a perfect square or perfect cube.

Therefore, there are numbers that are either a perfect square or perfect cube. Therefore, there are numbers that are neither a perfect square or a perfect cube.
Final answer
135