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jmc

number theory senior

Problem

Let How many positive integer divisors of are less than but do not divide ?
Solution
We know that must have factors by its prime factorization. If we group all of these factors (excluding ) into pairs that multiply to , then one factor per pair is less than , and so there are factors of that are less than . There are factors of , which clearly are less than , but are still factors of . Therefore, using complementary counting, there are factors of that do not divide .
Final answer
589