We call a positive integer good if there is a sequence of positive integers satisfying , and for all positive integers . Find the sum of all good numbers.
Solution
By the condition of the problem statement, we have . This is equivalent to . Let . Then we have . Therefore we have for all . We know that is a positive rational number because , and are all positive integers. We claim that must be an integer. Suppose that with . Since is always an integer for , we must have for all . This contradicts . Conversely, if is an integer, inductively all are integers. Now we compute , so is an integer. Therefore is an integer. Combining with the condition that is an integer limits the possible values of to , 224. The values all lead to for some whereas leads to a good sequence. The sum of the solutions is .
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