Find the minimum number of perfect cubes such that their sum is equal to .
Bonus: Can you find the numbers?
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Since a 3 ≡ 1 , 0 , − 1 ( m o d 9 ) and 3 4 6 3 4 6 ≡ 4 ( m o d 9 ) , it follows that 3 4 6 3 4 6 cannot be written as a sum of one, two or three cubes. On the other hand 3 4 6 3 4 6 = ( 3 4 6 1 1 5 × 7 ) 3 + ( 3 4 6 1 1 5 ) 3 + ( 3 4 6 1 1 5 ) 3 + ( 3 4 6 1 1 5 ) 3 using the fact that 3 4 6 = 7 3 + 1 3 + 1 3 + 1 3 , so the desired answer is 4 .