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jmc

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.
Final answer
1963