Consider all integers which satisfy the following condition:
, but for all , we have .
The smallest positive integer value of that satisfies this condition can be written in the form , where is a prime number. What is the value of ?
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If the equation 3 6 ! ≡ 0 ( m o d n ) becomes true only when we achieve the number 36! , this means that any divisors of n are being "taken away" at each factor of this number, and the last divisor (or divisors) is (are) taken on 3 6 = 2 2 × 3 2 . The divisor taken on 36! only can be 2 or 3 . The number of times which the factor 2 appears between 1 and 35 , can be counted by the intervals. Hence, we have:
2 [ 2 , 3 4 ] = 1 7
4 [ 4 , 3 2 ] = 8
8 [ 8 , 3 2 ] = 4
1 6 [ 1 6 , 3 2 ] = 2
3 2 3 2 = 1
1 7 + 8 + 4 + 2 + 1 = 3 2
3 [ 3 , 3 3 ] = 1 1
9 [ 9 , 2 7 ] = 3
2 7 2 7 = 1
1 1 + 3 + 1 = 1 5
When 36! takes away any divisor, n becomes 2 3 3 or 3 1 6 . Now, we only need to find which power is smaller. Two different numbers can only be smaller than another, equal to another or greater than another. We can use a question mark as operator, as follows:
2 3 3 ? 3 1 6
2 1 6 3 3 ? 3
2 2 × 2 1 6 1 ? 3
2 1 6 1 ? 4 3
As we know, any power of a number greater than one, is greater than one too. So, the left side of the comparation is greater than the right side.
2 3 3 > 3 1 6
n = a b = 3 1 6
And the sum is a + b = 3 + 1 6 = 1 9 .