Real-valued root

[Paul Royik]

I try to use the bridge between sympy and numpy, namely, the lambdify function.

It looks ok, but there is an issue when I try to calculate the values of x^(2/5) on the interval [-5,5]. 

Of course, for negative x, the value is nan, because, I guess, the root is not real-valued.

How to make it real-valued?

Thank you.

[peter.st...@gmail.com]

If I put it as (x^2)^(1/5) it works fine.

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[Paul Royik]

What is x^(3/5)?

[peter.st...@gmail.com]

I do not know.

What is x^(2/pi)?

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[Paul Royik]

I mean what if x^(3/5) ?

[peter.st...@gmail.com]

I do not know.

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[Oscar Benjamin]

You can use e.g.

In [22]: lambdify(x, (sign(x)*abs(x)**(1/5))**2)([-2, -1, 0, 1, 2])
Out[22]: array([1.31950791, 1. , 0. , 1. , 1.31950791])

The real_root function gives a more complicated form:

In [23]: real_root(x, 5)**2
Out[23]:
⎧ 2/5 2
⎪│x│ ⋅sign (x) for im(x) = 0
⎨
⎪ 2/5
⎩ x otherwise

That one works as well but prints out warnings when lambdified.

--
Oscar

[Paul Royik]

Thank you very much!