A recent episode of the popular YouTube channel "blackpenredpen" is about solutions to the famous equation

x^2 = 2^x

You should be able to see two solutions yourself, x = 2 and x = 4, because 2^2 = 2^2 and 4^2 = 2^4.

This post is about a third solution.

Contents

Two graphs

The figure shows the graphs of x^2 and 2^x and their intersections. The narrow silver area is the only region the right half plane where x^2 is greater than 2^x.

We can see there are three intersections -- the two we knew about at x = 2 and x = 4 and a third one at a negative value of x. Blackpen carefully explains how to characterize this negative solution.

   black_pen_plot

The negative solution

I was pleased to learn that our Symbolic Math Toolbox can find all three solutions, provided we indicate that we are only interested in real-valued solutions. (The behavior of x^2 = 2^x for complex x is a topic for another day.)

   syms x real
   z = solve(x^2 == 2^x)

 
z =
 
                                2
                                4
-(2*lambertw(0, log(2)/2))/log(2)
 

We want to know more about that third solution.

   z = z(3)

 
z =
 
-(2*lambertw(0, log(2)/2))/log(2)
 

The function lambertw(x) involved in this solution is an old friend, the LambertW function.

Logarithm

The blue curve in the following figure is a plot of exp(x). It goes to zero for negative x and grows exponentially for positive x. Imagine interchanging the x and y axes by reflecting the blue curve about the diagonal dashed line to produce the orange curve. The orange curve is familiar. It is a plot of the functional inverse of exp(x) which we know as log(x). If y = log(x) then x = exp(y).

   log_plot

Current plot released

LambertW

The blue curve in the following figure is now a plot of x*exp(x). Reflecting the blue curve about the diagonal produces the orange curve, a plot of the functional inverse of x*exp(x). This function is not as familiar as log(x). It is lambertw(x). If y = lambertw(x) then x = y*exp(y).

We need to evaluate y = lambertw(x) at x = log(2)/2. This is the black dot. Once we have x and y, the negative solution to our famous equation is simply z = -y/x.

   lambertw_plot

Current plot released

Many digits

The Symbolic Math Toolbox variable precision arithmetic, vpa, can produce the numeric value of the negative solution to any number of digits.

    z
    vpaz = vpa(z,75)

 
z =
 
-(2*lambertw(0, log(2)/2))/log(2)
 
 
vpaz =
 
-0.766664695962123093111204422510314848006675346669832058460884376935552795725
 

Fixed points

Here is your homework. Investigate the iteration:

x = sign(x)*2^(x/2)

Consider three situations:

Thanks

Thanks to Mark Round for suggesting only real solutions.

Code

The code for the figures is available.

black_pen_plot

log_plot

lambertw_plot

Get the MATLAB code

Published with MATLAB® R2020b