The problem is that assumptions on Function don't do anything
(https://github.com/sympy/sympy/issues/6494). If you want to create a
Function with assumptions, you'll need to subclass Function
explicitly, like
class f(Function):
is_real = True
Aaron Meurer
On Tue, Feb 24, 2015 at 4:01 PM, John Peterson <[email protected]> wrote:
>
>
> On Tuesday, February 24, 2015 at 2:55:28 PM UTC-7, Ondřej Čertík wrote:
>>
>> On Tue, Feb 24, 2015 at 2:44 PM, John Peterson <[email protected]> wrote:
>> >
>> >
>> > On Friday, February 13, 2015 at 1:18:56 PM UTC-7, chaowen guo wrote:
>> >>
>> >> I run the following code in ipython3 notebook:
>> >>
>> >> import sympy
>> >> x=sympy.symbols("x",real=True)
>> >> f=sympy.symbols("f",cls=sympy.Function,real=True)
>> >> sympy.Abs(f(x)).diff(x)
>> >>
>> >> and expect that the answer is just df(x)/dx. However, sympy still treat
>> >> the function f is a complex function.
>> >>
>> >> So how to tell sympy that f is a real function?
>> >
>> >
>> >
>> > I would also like to know if there's a way to do this... "real=True"
>> > does
>> > not seem to have any special meaning for Functions.
>> >
>> > Here's an equivalent test case:
>> >
>> >> #!/usr/bin/env python
>> >> from sympy import *
>> >> X = Symbol('X', real=True)
>> >> f = Function('f', real=True)(X)
>> >> g = Function('g', real=True)(X)
>> >> print diff (abs(f-g), X)
>> >
>> >
>> > The output is:
>> >
>> >> ((re(f(X)) - re(g(X)))*(re(Derivative(f(X), X)) - re(Derivative(g(X),
>> >> X)))
>> >> + (im(f(X)) - im(g(X)))*(im(Derivative(f(X), X)) - im(Derivative(g(X),
>> >> X))))/Abs(f(X) - g(X))
>> >
>> >
>> > Note that a possible workaround is with subs:
>> >
>> >> print diff (abs(f-g), X).subs([(im(f),0),
>> >> (im(g),0),
>> >> (re(f),f),
>> >> (re(g),g),
>> >> (re(Derivative(f, X)), Derivative(f,
>> >> X)),
>> >> (re(Derivative(g, X)), Derivative(g,
>> >> X))])
>> >
>> >
>> > Which prints:
>> >
>> >> (f(X) - g(X))*(Derivative(f(X), X) - Derivative(g(X), X))/Abs(f(X) -
>> >> g(X))
>>
>> We have an issue for what diff(abs(x), x) should be:
>>
>> https://github.com/sympy/sympy/issues/8502
>
>
> Yes, I came across this while searching for solutions to my issue. Very
> interesting!
>
> Actually I'm not too concerned about exactly what form of the derivative is
> used, only that re() and im() don't appear for Functions declared to be
> real.
>
>
>> and what you found seems like a bug. SymPy should be able to use the
>> fact that "f" is real.
>
>
> OK. If you want me to, I can move this over to an issue at GitHub...
>
>
>
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