MHB Plotting y=x^1/3 - Why Different Results?

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The discussion centers on the discrepancies observed when plotting the graph of y=x^(1/3) using different plotters. It highlights that while the plots agree for x > 0, they diverge for x < 0 due to the handling of fractional powers. One plotter may incorrectly apply logarithmic calculations, leading to undefined results for negative x, as logarithms of negative numbers do not exist. The conversation also notes that calculators often evaluate fractional powers without accommodating negative bases properly. Ultimately, the proper definition of x^(1/3) for negative x is contingent on the denominator being an odd integer, which is not universally supported by calculators.
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Dear all,

I was using the computer in order to plot the graph of

\[y=x^{\frac{1}{3}}=\sqrt[3]{x}\]

and two different plotters gave two different results. I don't understand why. Can you kindly explain ?

The results are:

View attachment 8447

View attachment 8448

Thank you !
 

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First, of course, those are the same for x> 0 just scaled differently. As for x< 0 it looks like the first plotter is using logarithms "unthinkingly" to calculate fractional powers and the logarithm of negative numbers do not exist.
 
Generally $x^{1/3}$ is considered to be undefined for negative $x$, although some books do effectively define it as $-(-x)^{1/3}$.

Reasons are:
  1. The power identity $a^{b\cdot c}=(a^b)^c$ breaks down. Consider:
    $$-1 = (-1)^{2/3\cdot 3/2} \ne ((-1)^{2/3})^{3/2} = 1^{3/2} = 1$$
  2. Calculators evaluate it as $x^{0.33333}$, which is undefined for negative x.
    Note that we can only define something like $x^{1/3}$ for negative x if the power is a fraction with an odd number in the denominator, but that is generally not supported by calculators.
 
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