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 On the first derivative of the n-th tetration of f(x) Luknik Junior Fellow Posts: 8 Threads: 2 Joined: Oct 2021 10/16/2021, 08:30 PM Hello! I would like to share with you my formula to compute the first derivative of the $$n$$-th tetration of $$f(x)$$. In particular we have the following theorem: Let $$f(x)$$ be a differentiable function and $$n \in\mathbb{N}$$ , $$n\geq 2$$. Hence: $$\frac{d}{dx}{^{n}f(x)}={^{n}f(x)}{^{n-1}f(x)}\frac{f'(x)}{f(x)}\Bigl\{\sum_{j=0}^{n-2}\Bigl\{\Bigl[\prod_{j}^{n-2}{^{j}f(x)}\Bigr]\Bigl[\log\Bigl(f(x)\Bigr)\Bigr]^{n-j-1}\Bigl\}+1\Bigr\}$$ For example 1 consider $$g(x)=x^{x^{x^{x}}}={^{4}x}$$. $$g'(x)=x^{x^{x^{x}}}x^{x^{x}}\frac{1}{x}\Bigl\{x^{x}x\Bigl[[\log(x)]^3+[\log(x)]^2\Bigr]+x^{x}\log(x)+1\Bigr\}$$ For example 2 consider $$h(x)=(\sin x)^{(\sin x)^{(\sin x)}}={^{3}\sin(x)}$$. $$h'(x)=(\sin x)^{(\sin x)^{(\sin x)}}(\sin x)^{(\sin x)}(\cot x)\Bigl\{(\sin x)[(\log(\sin x))^2+\log(\sin x)]+1\Bigr\}$$ I prove this by induction on $$n$$ , here below you can download the paper. Have a good day! Thank you for your attention. Attached Files   First derivative of a tetration.pdf (Size: 181.59 KB / Downloads: 64) Luca Onnis  tommy1729 Ultimate Fellow Posts: 1,493 Threads: 356 Joined: Feb 2009 10/26/2021, 10:05 PM Cool. Not sure if this is new. But somehow this must have applications ... regards tommy1729 Luknik Junior Fellow Posts: 8 Threads: 2 Joined: Oct 2021 10/27/2021, 06:11 AM I don't know! I didn't find anything like this online..I only found some examples for some specific cases but I have never found a generalization , but maybe I'm wrong. Anyway, maybe it could be possible to have a formula to compute the m-th derivative of the n-th tetration of a function, although it'll be very complicated to use. Who knows! Regards, Luknik Luca Onnis  Daniel Fellow Posts: 91 Threads: 33 Joined: Aug 2007 10/27/2021, 09:46 AM Howdy, You might be interested in my combinatoric solution to the m^th derivative of f^n(z). See https://www.tetration.org/Combinatorics/index.html . Daniel Luknik Junior Fellow Posts: 8 Threads: 2 Joined: Oct 2021 10/27/2021, 03:35 PM (This post was last modified: 10/27/2021, 03:46 PM by Luknik.) Thank you Daniel! I'll check it out.  I really hope that a closed formula for the m-th derivative of the n-th tetration of some functions exists. For example for $$f(x)=x$$. So..if $$g(x)={^{n}x}$$ Maybe we can find a closed formula for: $$\frac{d^{m}}{dx^{m}} {^{n}x}$$ where $$n,m\in\mathbb{N}$$ and $$n\geq 2$$ I have already found somethig interesting, I'll post some of the results I got. Thank you, regards. Luca Onnis  tommy1729 Ultimate Fellow Posts: 1,493 Threads: 356 Joined: Feb 2009 10/28/2021, 11:53 AM (10/27/2021, 03:35 PM)Luknik Wrote: Thank you Daniel! I'll check it out.  I really hope that a closed formula for the m-th derivative of the n-th tetration of some functions exists. For example for $$f(x)=x$$. So..if $$g(x)={^{n}x}$$ Maybe we can find a closed formula for: $$\frac{d^{m}}{dx^{m}} {^{n}x}$$ where $$n,m\in\mathbb{N}$$ and $$n\geq 2$$ I have already found somethig interesting, I'll post some of the results I got. Thank you, regards. That would be interesting , in combinations with ideas like continuum sum etc. regards tommy1729 « Next Oldest | Next Newest »

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