In a row with cells from left to right, some are coloured red, and others are coloured white. Let be the number of triples such that are white, is red, and are ordered from left to right (doesn't have to be right next to the other cells) in this order. Find the maximum value of .
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Let A n be 0 if the colour of the n t h cell from the left is red or otherwise 1 where 1 ≤ m , n ≤ 1 5 0 and S = i = 1 ∑ 1 5 0 A i
Lemma: ( 1 − A i ) k = ( 1 − A i ) ( k + A i ) Proof: R . H . S . − L . H . S . = ( 1 − A i ) ( k + A i ) − ( 1 − A i ) k = ( 1 − A i ) A i As A i = 0 o r 1 , R . H . S . − L . H . S . = 0 so L . H . S . = R . H . S . .
Then T = i = 1 ∑ 1 5 0 ( ( 1 − A i ) j = 1 ∑ i − 1 A j j = i + 1 ∑ 1 5 0 A j ) ≤ i = 1 ∑ 1 5 0 ( ( 1 − A i ) ( 2 ∑ j = 1 i − 1 A j + ∑ j = i + 1 1 5 0 A j ) 2 ) = i = 1 ∑ 1 5 0 ( 1 − A i ) ( 2 S ) 2 = ( 1 5 0 − S ) ( 2 S ) ( 2 S ) ≤ ( 3 ( 1 5 0 − S ) + 2 S + 2 S ) 3 = 5 0 3 = 1 2 5 0 0 0
Here is an example of how could we colour to attain T = 1 2 5 0 0 0 . We could colour the 5 0 cells from left and 5 0 cells from right white, the remaining 5 0 red.