Inspired by Otto Bretscher

Algebra Level 4

x x . . x π = π \huge x^{x^{.^{.^{x^{\pi}}}}}=\pi

Find all positive solutions x x of the equation above.

Notation: π = 3.14159... \pi=3.14159...


Inspiration

This question is part of the set All-Zebra

No solution exists π π \sqrt[\pi]{\pi} More than one solution exists. π \sqrt[\infty]{\pi} π \sqrt{\pi} None of the given options.

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1 solution

It is given that x x x π = π \large{x^{x^{\cdot^{\cdot^{x^\pi}}}}} = \pi . Now, if a real value x x satisfying the equation exists, then we have:

x π = π x = π π \begin{aligned} x^\pi & = \pi \\ \implies x & = \sqrt[\pi]{\pi} \end{aligned}

Now, if we consider a 0 = x π a_0 = x^\pi and a n + 1 = x a n a_{n+1} = x^{a_n} for n 1 n \ge 1 , then lim n a n = x x x π \displaystyle \lim_{n \to \infty} a_n = \large{x^{x^{\cdot^{\cdot^{x^\pi}}}}} . We note that if x = π π x= \sqrt[\pi]{\pi} then a 0 = ( π π ) π = π a_0 = (\sqrt[\pi]{\pi})^\pi = \pi and then a 1 = a 2 = a 3 = = π a_1 = a_2 = a_3 = \cdots = \pi , that is lim n a n = x x x π = π \displaystyle \lim_{n \to \infty} a_n = \large{x^{x^{\cdot^ {\cdot^{x^\pi}}}}} = \pi . But if x π π x \ne \sqrt[\pi]{\pi} , lim n a n \displaystyle \lim_{n \to \infty} a_n does not converge and it has no solution. Therefore, the only solution is x = π π x = \boxed{\sqrt[\pi]{\pi}} .

Elegant! :), a little typo in the fourth row. Explained it nicely and easily. :(+1):

Abhay Tiwari - 5 years, 1 month ago

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Thanks, I have amended the typo.

Chew-Seong Cheong - 5 years, 1 month ago

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Sir, l o g π x = 1 π log_{π} x=\frac{1}{π} , please check :).

Abhay Tiwari - 5 years, 1 month ago

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@Abhay Tiwari Still thinking about your previous problem.

Chew-Seong Cheong - 5 years, 1 month ago

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@Chew-Seong Cheong Which one sir? :curious face:

Abhay Tiwari - 5 years, 1 month ago

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@Abhay Tiwari One with 16 16 in place of π \pi .

Chew-Seong Cheong - 5 years, 1 month ago

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@Chew-Seong Cheong Okay, it's similar to this one sir. ;)

Abhay Tiwari - 5 years, 1 month ago

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