An Earnest Challenge

There appears to be a lot of expertise amongst the members of this site regarding solving nested radicals, so I thought I'd share a challenging one for your radical enjoyment:

f(x)=x+x2+x4+x8+x16+x32+......f(x) = \displaystyle\sqrt{x + \sqrt{\frac{x}{2} + \sqrt{\frac{x}{4} + \sqrt{\frac{x}{8} + \sqrt{\frac{x}{16} + \sqrt{\frac{x}{32} + ......}}}}}}.

The hope is that there is an exact solution, if only for f(1)f(1) if not for f(x)f(x) in general. I suppose one interesting feature of this function is that (f(x))2x=f(x2)(f(x))^{2} - x = f(\frac{x}{2}). I'm sure that there are many more interesting features waiting to be discovered.

Note by Brian Charlesworth
5 years, 1 month ago

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f(x) = 2sqrt(x)

Coby Tran - 5 years ago

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For higher degree of radicals f(x)= nth root of 2x

Amit Tripathi - 5 years ago

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Could it be written in this fashion?

f(x)=i=0f(x)2if(x) = \sqrt{\sum_{i=0} \frac{f(x)}{2^{i}}}

Giovanni GuessWhat - 5 years, 1 month ago

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no.. I think the 'x' terms under summation are missing and 'i' should start from 1 instead of 0.. if I'm not wrong.

Saptakatha Adak - 5 years, 1 month ago

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It's a recursive function. I thought "i" should start from 0 since the first term is "x", not "x/2"

Giovanni GuessWhat - 5 years, 1 month ago

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Your problem does not have closed form but

x+x2+x4+x16+x256+\sqrt{ x + \sqrt{\frac{x}{2} + \sqrt{\frac{x}{4} + \sqrt{\frac{x}{16} + \sqrt{\frac{x}{256} + \ldots}}}}}

Can have a closed form

Krishna Sharma - 5 years, 1 month ago

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It does? Cool. I'll have to figure out what that is, then.

Brian Charlesworth - 5 years, 1 month ago

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I was solving your radical and did a mistake and I solved the above radical :p, now I will post a problem on this ;)

Krishna Sharma - 5 years, 1 month ago

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@Krishna Sharma Haha. Well, a lot of "mistakes" have led to interesting discoveries. Ill keep an eye out for your problem.

Brian Charlesworth - 5 years, 1 month ago

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My bet is that there isn't any closed form expression for this, not even in the special case of x=1x=1.

Michael Mendrin - 5 years, 1 month ago

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You're probably right, but I'm getting used to seeing rabbits being pulled out of hats so I thought I'd post the problem just in case.

Brian Charlesworth - 5 years, 1 month ago

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