A string of pearls

Algebra Level 5

Consider an infinite sequence { a 1 , , a n , } \{a_1,\ldots,a_n,\ldots\} such that J = a 1 ( 1 a 2 ) = a 2 ( 1 a 3 ) = J=a_1\left(1-a_2\right)=a_2\left(1-a_3\right)=\ldots

If J = 3 20 \displaystyle J=\frac{3}{20} , find the maximum value of a 2014 a_{2014} .


The answer is 0.816228.

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1 solution

a 2 = 1 J a 1 a 3 = 1 J a 2 = 1 J 1 J a 1 \begin{aligned} a_2&=1-\frac{J}{a_1} \\ a_3&=1-\frac{J}{a_2}=1-\frac{J}{1-\frac{J}{a_1}} \end{aligned}

Equivalently, we can write a n = 1 J a n a_n=1-\frac{J}{a_n}

So in order to get a 2014 a_{2014} to be maximum we can take maximum possible value for a n a_n possible.

so if we extrapolate it to infinity we get,

a = 1 ( J / ( 1 J / ( 1 J infinite terms ) ) ) ) ) a_{\infty}=1-(J/(1-J/(1-J\ldots \text{ infinite terms})))))

Let the value be x x so, we get

x = 1 3 / 20 x x=1-3/20x 20 x 2 20 x + 3 = 0 \rightarrow 20x^2-20x+3=0

solving for x x , we get x = ( 5 + 10 ) / 10 x=(5+\sqrt{10})/10 and 5 = ( 10 ) ) / 10 5=\sqrt(10))/10

for maximum value we take ( 5 + 10 ) / 10 = 0.816 (5+\sqrt{10})/10=0.816

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