Complex Generalisation of Infinite Tetration

Algebra Level 4

Tetration is defined as

n a = a a a n \large {^{n}a} = \underbrace{a^{a^{\cdot^{\cdot^{a}}}}}_n

If z z is a complex number, then find the value of

z {^{\infty}z}

Details and Assumptions :

W W is the Lambert's W function. It is defined as" x = W ( x ) e W ( x ) x = W(x) e^{W(x)} for all complex number x x .

W ( ln ( z ) ) ln ( z ) \dfrac {W(- \ln (z))}{- \ln (z)} W ( ln ( z ) ) ln ( z ) \dfrac{W(- \ln (z))}{\ln (z)} W ( ln ( z ) ) ln ( z ) \dfrac {W(\ln (z))}{- \ln (z)} W ( ln ( z ) ) ln ( z ) \dfrac {W(\ln (z))}{\ln (z)}

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1 solution

Hasan Kassim
Oct 22, 2014

Using substitutions and rearrangements, we are going to write y = z \displaystyle y= ^{\infty} z in the form of x = W ( x ) e W ( x ) \displaystyle x=W(x)e^{W(x)} (So we are looking after y y ).

Since we have an infinite tetration : y = z = z z z . . z = z y \displaystyle y= ^{\infty} z = z^{z^{z^{.^{.^{z}}}}} = z^y

y = z y = e y ln z < = > y e y ln z = 1 \displaystyle y=z^y = e^{y\ln z} <=> ye^{-y\ln z} =1

Multiply both sides by ln z -\ln z :

( y ln z ) e y ln z = ln z \displaystyle (-y\ln z)e^{-y\ln z} = -\ln z

now define x = ln z x= -\ln z and f ( x ) = y ln z f(x) = -y\ln z

= > x = f ( x ) e f ( x ) \displaystyle => x=f(x)e^{f(x)} therefore f ( x ) = W ( x ) f(x) = W(x)

now substitute back x x and f ( x ) f(x) :

y ln z = W ( ln z ) = > y = W ( ln z ) ln z \displaystyle -y\ln z = W(-\ln z) =>\boxed{ y= \frac{W(-\ln z)}{-\ln z}}

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