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Aug 10, 2015
 #5
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x^x=2.  How to solve this equation?

 

$$\small{\text{$
\begin{array}{rcl}
x^x &=&2 \qquad | \qquad \ln{()}\\
x\ln(x) &=& \ln{(2)} \qquad | \qquad x = e^{ln{(x)}}\\
e^{ln{(x)}}\cdot \ln{(x)} & =& \ln{(2)} \qquad | \qquad z = \ln{(x)} \\
e^z\cdot z &=& \ln{(2)}\\
\end{array}
$}}\\\\
\small{\text{
If we have $\boxed{~a=z\cdot e^z~}$,
the solution is $\boxed{~z=W(a)~}$. and $\boxed{~x=e^z~} $}}\\
\small{\text{
In mathematics, the Lambert W function, is also called the omega function or product logarithm
}}$$

$$\small{\text{$
\begin{array}{lrcl}
\text{We have }& z &=& \ln{(x)}\\
\text{and } & e^z\cdot z &=& \ln{(2)}\\
\text{so } & z &=& W(\ln{(2)})\\
\text{and } & \ln{(x)}&=& W(\ln{(2)})\\\\
& e^{\ln{(x)}} &=& e^{W(\ln{(2)})}\\
\text{finally }& x &=& e^{W(\ln{(2)})}\\
\text{also } & \mathbf{ x }& \mathbf{=} & \mathbf{\dfrac{\ln{(2)}}{W(\ln{(2)})} }
\end{array}
$}}\\\\$$

$$\small{\text{ Numerical Evaluation:}}\\ \small{\text{ The $W$ function may be approximated using Newton's method, with successive approximations to}}\\ \small{\text{ $z=W(a)$ (so $a=ze^z$) being $z_{j+1}=z_j-\dfrac{z_j e^{z_j}-a}{e^{z_j}+z_j e^{z_j}}$. }}\\
\small{\text{We start the Iteration with $z_0 = 1$}}\\$$

 

$$\small{\text{$
\begin{array}{rclcrcl}
z_1 &=& 1 - \dfrac{ 1\cdot e^1 - \ln{(2)} }{e^1+ 1\cdot e^1 } = 0.62749729872 &\quad & x_1 &=& \dfrac{ \ln{(2)} } {z_1} = 1.10462177603\\\\
z_2 &=& z_1-\dfrac{z_1 e^{z_1}- \ln{(2)} }{e^{z_1}+z_1 e^{z_1}} = 0.24045491796 &\quad & x_2 &=& \dfrac{ \ln{(2)} } {z_2} = 2.88264921519\\\\
z_3 &=& z_2-\dfrac{z_2 e^{z_2}- \ln{(2)} }{e^{z_2}+z_2 e^{z_2}} = 0.48596644327 &\quad & x_3 &=& \dfrac{ \ln{(2)} } {z_3} = 1.42632724987\\\\
z_4 &=& z_3-\dfrac{z_3 e^{z_3}- \ln{(2)} }{e^{z_3}+z_3 e^{z_3}} = 0.44585119296 &\quad & x_4 &=& \dfrac{ \ln{(2)} } {z_4} = 1.55466036989\\\\
z_5 &=& z_4-\dfrac{z_4 e^{z_4}- \ln{(2)} }{e^{z_4}+z_4 e^{z_4}} = 0.44443778365 &\quad & x_5 &=& \dfrac{ \ln{(2)} } {z_5} = 1.55960452971\\\\
z_6 &=& z_5-\dfrac{z_5 e^{z_5}- \ln{(2)} }{e^{z_5}+z_5 e^{z_5}} = 0.44443609102 &\quad & x_6 &=& \dfrac{ \ln{(2)} } {z_6} = 1.55961046945\\\\
z_7 &=& z_6-\dfrac{z_6 e^{z_6}- \ln{(2)} }{e^{z_6}+z_6 e^{z_6}} = 0.44443609102 &\quad & x_7 &=& \dfrac{ \ln{(2)} } {z_7} = 1.55961046946\\\\
\end{array}
$}}$$

 

x = 1.55961046946

 

Aug 10, 2015
 #1
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0
Aug 10, 2015
 #2
avatar+4 
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Aug 10, 2015
 #1
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Aug 10, 2015
 #11
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Aug 10, 2015

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