A Famous Equation, x^2 = 2^x
最近的流行YouTube频道的一集"blackpenredpen"关于著名方程的解决金宝搏官方网站方案
x^2 = 2^x
您应该能够自己看到两种解决方案,金宝搏官方网站X= 2和X= 4, 因为2^2 = 2^2和4^2 = 2^4。
This post is about a third solution.
内容
两个图
The figure shows the graphs ofX^2和2^x和their intersections. The narrow silver area is the only region the right half plane whereX^2大于2^x。
我们可以看到有三个交叉路口 - 我们知道的两个交叉点X= 2和X= 4和a third one at a negative value ofX。黑彭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 ofx^2 = 2^x用于复杂X是另一天的话题。)
symsX真实的z = solve(x^2 == 2^x)
z = 2 4 -(2*lambertw(0, log(2)/2))/log(2)
我们想了解有关第三个解决方案的更多信息。
z = z(3)
z = - (2*lambertw(0,log(2)/2))/log(2)
功能兰伯特(X)involved in this solution is an old friend,兰伯特功能。
对数
下图中的蓝色曲线是exp(x)。负数为零X并成倍增长以呈阳性X。想象一下,通过反射对角线虚线的蓝色曲线来互换X和Y轴以产生橙色曲线。橙色曲线很熟悉。它是功能逆的图exp(x)我们知道log(x)。如果y= log(x)然后x = exp(y)。
log_plot
当前释放的情节
兰伯特
下图中的蓝色曲线现在是x*exp(x)。Reflecting the blue curve about the diagonal produces the orange curve, a plot of the functional inverse ofx*exp(x)。此功能不像log(x)。这是兰伯特(X)。如果y =兰伯特(x)然后x = y*exp(y)。
我们需要评估y =兰伯特(x)在x = log(2)/2。这是黑点。一旦我们有X和y,对我们著名方程式的负面解决方案简直就是z = -y/x。
lambertw_plot
当前释放的情节
许多数字
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 = -0.76666646959621230931120442510314484848484848484848484848484848466666666666698320584608460846084693555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555555太样
固定点
Here is your homework. Investigate the iteration:
x =标志(x)*2^(x/2)
考虑三种情况:
- 启动值在0到4之间。
- 启动值大于4。
- 启动值小于0。
Thanks
Thanks to Mark Round for suggesting only real solutions.
代码
可以使用这些数字的代码。
注释
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