Ramanujan's nested radical is great! (3)

Algebra Level 3

k = 2 2017 2016 2016 2016 2016 k k k k k = exp 2016 ( n = 1 x 1 n ) \large \displaystyle \prod_{k=2}^{2017}\sqrt[k]{2016\sqrt[k]{2016\sqrt[k]{2016\sqrt[k]{2016\sqrt[k]{\cdots}}}}}=\exp_{2016} \left( \sum_{n=1}^x \dfrac1n\right)

Find the value of x x satisfying the equation above.

Notation : exp A ( B ) \exp_A (B) denotes the exponential function A B A^B .


The answer is 2016.

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2 solutions

Jeko Lonardo
Jun 29, 2016

Let: a a = = 2016 2016 2016 . . . k k k k \sqrt [k]{2016\sqrt [k]{2016\sqrt [k]{2016\sqrt [k]{...}}}}

a k a^k = = 2016 2016 2016 2016 . . . k k k \sqrt [k]{2016\sqrt [k]{2016\sqrt [k]{...}}}

a k a^k = = 2016 2016 a a

a k 1 a^{k-1} = = 2016 2016

a a = = 2016 k 1 \sqrt [k-1]{2016}

k = 2 2017 \prod_{k=2}^{2017} 2016 k 1 \sqrt [k-1]{2016} = = 2016 1 \sqrt [1]{2016} 2016 2 \sqrt [2]{2016} 2016 3 \sqrt [3]{2016} *... 2016 2016 \sqrt [2016]{2016}

= = 201 6 1 + 1 2 + 1 3 + . . . + 1 2016 2016^{1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2016}}

= = 201 6 n = 1 2016 1 n 2016^{\sum_{n=1}^{2016} \frac{1}{n}}

T h u s , x = 2016 Thus, x=2016

Tommy Li
Jun 29, 2016

1 + 1 n + 1 n 2 + = 1 1 1 n 1+\dfrac{1}{n}+\dfrac{1}{n^2}+\dots=\dfrac{1}{1-\frac{1}{n}}

1 + 1 n + 1 n 2 + = 1 + 1 n 1 1+\dfrac{1}{n}+\dfrac{1}{n^2}+\dots=1+\dfrac{1}{n-1}

1 n + 1 n 2 + 1 n 3 + = 1 n 1 \dfrac{1}{n}+\dfrac{1}{n^2}+\dfrac{1}{n^3}+\dots=\dfrac{1}{n-1}

x 1 n 1 = x 1 n + 1 n 2 + 1 n 3 + \Rightarrow \large x^{\frac{1}{n-1}}=x^{\frac{1}{n}+\frac{1}{n^2}+\frac{1}{n^3}+\dots}

x 1 n 1 = x 1 n ( 1 + 1 n ( 1 + 1 n ( 1 + ) ) \Rightarrow \large x^{\frac{1}{n-1}}=x^{\frac{1}{n}(1+\frac{1}{n}(1+\frac{1}{n}(1+\dots))}

x 1 n 1 = x x x . . . n n n n \Rightarrow \large x^{\frac{1}{n-1}}=\sqrt[n]{x\sqrt[n]{x\sqrt[n]{x\sqrt[n]{...}}}}

k = 2 2017 2016 2016 2016 2016 . . . k k k k k = k = 2 2017 201 6 1 k 1 \Rightarrow \displaystyle \prod_{k=2}^{2017}\sqrt[k]{2016\sqrt[k]{2016\sqrt[k]{2016\sqrt[k]{2016\sqrt[k]{...}}}}}=\displaystyle \prod_{k=2}^{2017}2016^{\frac{1}{k-1}}

k = 2 2017 201 6 1 k 1 \displaystyle \prod_{k=2}^{2017}2016^{\frac{1}{k-1}}

= 201 6 1 + 1 2 + 1 3 + + 1 2016 = 2016^{1+\frac{1}{2}+\frac{1}{3}+\dots+\frac{1}{2016}}

= 201 6 ( n = 1 2016 1 n ) =2016^{\left(\displaystyle \sum_{n=1}^{2016}\frac{1}{n}\right)}

x = 2016 \Rightarrow x=2016

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