Which is the largest positive integer such that it is divisible by every integer in the range ?
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Let n be such a number (not necessarily the largest, but, being divisible by every number from 2 to ⌊ n ⌋ .
Let m be the largest integer less than or equal to n . Then m 2 ≤ n < ( m + 1 ) 2 .
Since, n is a multiple of m and should be within this range, we would have the choices of n being restricted to n = { m 2 , m ( m + 1 ) , m ( m + 2 ) } .
However, n should also be divisible by ( m − 1 ) . Hence, n ≡ 0 m o d ( m − 1 )
Considering the three cases :
If n = m 2 , m 2 m o d ( m − 1 ) = ( m m o d ( m − 1 ) ) 2 = 1 ≡ 0 m o d ( m − 1 ) which means that m − 1 = 1 ⟹ m = 2 and n = 4 .
If n = m ( m + 1 ) , m ( m + 1 ) m o d ( m − 1 ) = 1 × 2 = 2 ≡ 0 m o d ( m − 1 ) which means that m − 1 = 1 , 2 ⟹ m = 2 , 3 and n = 6 , 1 2 .
If n = m ( m + 2 ) , m ( m + 2 ) m o d ( m − 1 ) = 1 × 3 = 3 ≡ 0 m o d ( m − 1 ) which means that m − 1 = 1 , 3 ⟹ m = 2 , 4 and n = 8 , 2 4 .
These are the only choices for n . Of these the maximum is 2 4 .