An integer n is written on a board. Then, in every minute, if n = a + b , then n is replaced by a b .
Determine the number of positive integers n ≤ 1 0 0 0 such that the number 2 0 1 3 can be attained.
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Note that 2 0 1 3 = 3 ⋅ 1 1 ⋅ 6 1 = 3 3 ⋅ 6 1 , so 3 3 + 6 1 = 9 4 can make 2013.
Also, if n > 4 , then 2 ( n − 2 ) = 2 n − 4 > n is achievable, so n can be increased arbitrarily.
In addition, 1 ( n − 1 ) = n − 1 < n is achievable, so n can be decreased arbitrarily.
Then, any 4 < n ≤ 1 0 0 0 works. Since 1,2,3,4 cannot be increased past themselves, the answer is 9 9 6 .
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2 0 1 3 = 3 × 1 1 × 6 1 = 3 3 × 6 1
So, if we can attain at least 9 4 then we are done on the next minute . Confused with at least ? Just note that from n + 1 we can easily attain n . Thus, we have to rule out positive integers from which we can't go greater than 9 3 .
Consider n ≤ 4 . For this case we can't get integers greater than 4 . And for every n ≥ 5 we can hop and jump to reach integers greater than 9 3 and then 9 4 by the method . It concludes that we can attain 2 0 1 3 if 5 ≤ n ≤ 1 0 0 0 .So, the answer is 1 0 0 0 − 4 = 9 9 6