Red and White Cells 2

Algebra Level 4

In a row with 150 150 cells from left to right, some are coloured red, and others are coloured white. Let T T be the number of triples ( C 1 , C 2 , C 3 ) (C_1,C_2,C_3) such that C 1 , C 3 C_1, C_3 are white, C 2 C_2 is red, and C 1 , C 2 , C 3 C_1, C_2, C_3 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 T T .

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The answer is 125000.

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1 solution

Culver Kwan
Mar 9, 2020

Let A n A_{n} be 0 0 if the colour of the n t h n^{th} cell from the left is red or otherwise 1 1 where 1 m , n 150 1\le m,n\le150 and S = i = 1 150 A i S=\sum^{150}_{i=1}A_i

Lemma: ( 1 A i ) k = ( 1 A i ) ( k + A i ) (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 \begin{aligned}R.H.S.-L.H.S.&=(1-A_i)(k+A_i)-(1-A_i)k\\&=(1-A_i)A_i\end{aligned} As A i = 0 o r 1 A_i=0 or 1 , R . H . S . L . H . S . = 0 R.H.S.-L.H.S.=0 so L . H . S . = R . H . S . L.H.S.=R.H.S. .

Then T = i = 1 150 ( ( 1 A i ) j = 1 i 1 A j j = i + 1 150 A j ) i = 1 150 ( ( 1 A i ) ( j = 1 i 1 A j + j = i + 1 150 A j 2 ) 2 ) = i = 1 150 ( 1 A i ) ( S 2 ) 2 = ( 150 S ) ( S 2 ) ( S 2 ) ( ( 150 S ) + S 2 + S 2 3 ) 3 = 5 0 3 = 125000 \begin{aligned}T&=\sum^{150}_{i=1}\bigg((1-A_i)\sum^{i-1}_{j=1}A_j\sum^{150}_{j=i+1}A_j\bigg)\\&\le\sum^{150}_{i=1}\Bigg(\Big(1-A_i)(\frac{\sum^{i-1}_{j=1}A_j+\sum^{150}_{j=i+1}A_j}2\Big)^2\Bigg)\\&=\sum^{150}_{i=1}(1-A_i)\bigg(\frac{S}{2}\bigg)^2\\&=(150-S)\bigg(\frac{S}{2}\bigg)\bigg(\frac{S}{2}\bigg)\\&\le\Bigg(\frac{(150-S)+\frac{S}{2}+\frac{S}{2}}{3}\Bigg)^3\\&=50^3\\&=125000\end{aligned}

Here is an example of how could we colour to attain T = 125000 T=125000 . We could colour the 50 50 cells from left and 50 50 cells from right white, the remaining 50 50 red.

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