2012 AMC 12B Problems/Problem 24
Define the function on the positive integers by setting and if is the prime factorization of , then For every , let . For how many s in the range is the sequence unbounded?
Note: A sequence of positive numbers is unbounded if for every integer , there is a member of the sequence greater than .
First of all, notice that for any odd prime , the largest prime that divides is no larger than , therefore eventually the factorization of does not contain any prime larger than . Also, note that , when it stays the same but when it grows indefinitely. Therefore any number that is divisible by or any number such that is divisible by makes the sequence unbounded. There are multiples of within . also works: .
Now let's look at the other cases. Any first power of prime in a prime factorization will not contribute the unboundedness because . At least a square of prime is to contribute. So we test primes that are less than :
works, therefore any number that are divisible by works: there are of them.
could also work if satisfies , but .
does not work.
works. There are no other multiples of within .
could also work if , but already.
For number that are only divisible by , they don't work because none of these primes are such that could be a multiple of nor a multiple of .
In conclusion, there are number of 's ... .
Video Solution by Richard Rusczyk
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