Re: [sympy] diff(x^x,x)

[Oscar Benjamin]

On 29 May 2015 22:06, "Thomas Leitz" <unruh...@gmail.com> wrote:
>
> Hi,
>
> the online shell at http://live.sympy.org/ tells me
>
> diff(x^x,x) = 0
>
> Why is that?

^ doesn't do what you're expecting in Python. Use ** for exponentiation.

--
Oscar

[Francesco Bonazzi]

In [1]: x^x
Out[1]: False

In [2]: diff(False, x)
Out[2]: 0

You should use ** instead of ^ if you'd like to represent exponentiation. The symbol ^ stands for XOR.

[Christophe Bal]

And because  x^x  means  x xor x  which numeric value is  0  or  1  so the derivative is equal to zero.

Christophe BAL

Enseignant de mathématiques en Lycée et développeur Python amateur

---

French math teacher in a "Lycée" and Python amateur developer

[Aaron Meurer]

Actually, that's not quite true. SymPy has its own boolean types, and
x ^ x gives the SymPy BooleanFalse (S.false). The derivative of this
is 0 not because it interprets it as a numeric 0 but because it
doesn't depend on x, and diff automatically knows that the derivative
of any expression that doesn't depend on x is 0.

Perhaps we should make derivatives of booleans give TypeError. That
would make it much easier to see what is going on.

It may also be worth it to print a warning in SymPy Live when ^ is used.

Aaron Meurer

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[Joachim Durchholz]

Am 31.05.2015 um 22:50 schrieb Aaron Meurer:
> Perhaps we should make derivatives of booleans give TypeError. That
> would make it much easier to see what is going on.

+1

Similarly, integer-valued functions should (probably?) emit a warning if
a derivative is taken. The derivative is technically correct, but
usually not what one wanted.

> It may also be worth it to print a warning in SymPy Live when ^ is used.

I'll be all for it if somebody picks that up.

[Christophe Bal]

I also vote for the TypeError.

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