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counting and probability intermediate
Problem
How many positive integers less than 2008 have an even number of divisors?
Solution
A number has an odd number of divisors if and only if it is a perfect square. To see this, notice that the divisors and pair up, except for when , or .
Therefore, the only integers not counted are the perfect squares. Since and , there are 44 positive integers less than 2008 with an odd number of divisors, leaving positive integers less than 2008 with an even number of divisors.
Therefore, the only integers not counted are the perfect squares. Since and , there are 44 positive integers less than 2008 with an odd number of divisors, leaving positive integers less than 2008 with an even number of divisors.
Final answer
1963