Exponential issues 02

Calculus Level 3

If x x is a positive real number and m m is the minimum value of x x x x x^{x^{x^{x}}} .

Find the value of m m to 3 significant digits.


The answer is 0.593.

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1 solution

x x x x x x x x x ( x x x 1 + x x x log ( x ) ( x x 1 + x x log ( x ) ( log ( x ) + 1 ) ) ) \frac{\partial x^{x^{x^x}}}{\partial x} \Rightarrow x^{x^{x^x}} \left(x^{x^x-1}+x^{x^x} \log (x) \left(x^{x-1}+x^x \log (x) (\log (x)+1)\right)\right)

Numerically searching for a root of that expression, that is, searching for a zero value for that expression, near x being 0.25, gives x = 0.274689385244017 x=0.274689385244017 . Evaluating x x x x x^{x^{x^x}} at x = 0.274689385244017 x=0.274689385244017 gives 0.593237297769728.

Evaluating x x x x x x \frac{\partial \frac{\partial x^{x^{x^x}}}{\partial x}}{\partial x} , x x x x ( x x x 1 + x x x log ( x ) ( x x 1 + x x log ( x ) ( log ( x ) + 1 ) ) ) 2 + x x x x ( x x x 1 ( x x 1 x + x x log ( x ) ( log ( x ) + 1 ) ) + x x x 1 ( x x 1 + x x log ( x ) ( log ( x ) + 1 ) ) + x x x log ( x ) ( x x 1 + x x log ( x ) ( log ( x ) + 1 ) ) 2 + x x x log ( x ) ( x x 1 log ( x ) + x x 1 ( log ( x ) + 1 ) + x x 1 ( x 1 x + log ( x ) ) + x x log ( x ) ( log ( x ) + 1 ) 2 ) ) x^{x^{x^x}} \left(x^{x^x-1}+x^{x^x} \log (x) \left(x^{x-1}+x^x \log (x) (\log (x)+1)\right)\right)^2+ x^{x^{x^x}} \left(x^{x^x-1} \left(\frac{x^x-1}{x}+x^x \log (x) (\log (x)+1)\right)+ x^{x^x-1} \left(x^{x-1}+x^x \log (x) (\log (x)+1)\right)+x^{x^x} \log (x) \left(x^{x-1}+ x^x \log (x) (\log (x)+1)\right)^2+x^{x^x} \log (x) \left(x^{x-1} \log (x)+ x^{x-1} (\log (x)+1)+x^{x-1} \left(\frac{x-1}{x}+\log (x)\right)+x^x \log (x) (\log (x)+1)^2\right)\right) , at x = 0.274689385244017 x=0.274689385244017 gives 3.66815838086362, which is a positive number and therefore the root is a minimum.

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