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Calculate the product of two radicals?

I used Basic Math Assistant to calculate the product of two radicals:

In[24]:= Power[x, (3)^-1] Power[x^2, (3)^-1]

Out[24]= x^(1/3) (x^2)^(1/3)

enter image description here

It's obvious that the result must be simplified to x. Unfortunately, Mma didn't provide such simplification...

Then, I calculated the same product as the product of two powers:

In[25]:= x^(2/3)*x^(1/3)

Out[25]= x

The result is that it must be expected.

Why the pallet application produces such an issue?

12 Replies

Thank you, Sander! It's interesting to observe that Mma has slightly different images for (Power) radicals and (Surd) radicals:

enter image description here

Also read:

tutorial/FunctionsThatDoNotHaveUniqueValues

that you can find in the documentation.

POSTED BY: Sander Huisman

Use CubeRoot or Surd to get the real-valued roots

POSTED BY: Sander Huisman

The graph of cubic root from Wikipedia:

enter image description here

The graph of cubic root from Mathematica:

enter image description here

Indeed, not a problem, a feature! Squaring and taking a cube root simply do not commute. Commutator is not equal to 0.

POSTED BY: Sander Huisman

Branch cuts are not a problem, they are a feature of mathematics.

POSTED BY: Frank Kampas

The problem appears because of the writing form of the second radical:

enter image description here

Evidently, it's not a problem to obtain the correct result. But, Mma submit a strange result, especially for some users, e.g. school students.

It's interesting to see the full form of the expression:

enter image description here

In my opinion, the result have to be more rigorous, e.g. to highlight correct result for Reals and Complex numbers, separately.

In[5]:= Reduce[Power[x, (3)^-1] Power[x^2, (3)^-1] == x, x, Reals]

Out[5]= x >= 0
POSTED BY: Frank Kampas

How about :

Simplify[Power[x, (3)^-1] Power[x^2, (3)^-1], x > 0]
(* x *)
POSTED BY: Mariusz Iwaniuk

You can however use

PowerExpand[x^(1/3) ((x^2)^(1/3))]

to simplify such kind of expressions, but be aware it might not give the right result always:

PowerExpand converts (a b)^c to a^cb^c, whatever the form of c is.

PowerExpand also converts (a^b)^c to a^(b c), whatever the form of c is.

The transformations made by PowerExpand are correct in general only if c is an integer or a and b are positive real numbers.

POSTED BY: Sander Huisman

Unfortunately this can not be simplified without getting into trouble for some numbers:

x^(1/3) ((x^2)^(1/3))
% /. x -> -1.0 - I

or, without complex numbers, just negative numbers:

x^(1/3) ((x^2)^(1/3))
% /. x -> -2.0
POSTED BY: Sander Huisman
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