On Wed, Oct 1, 2008 at 10:50 PM, william ratcliff
<[EMAIL PROTECTED]> wrote:
> I believe there is a glitch in evalf when complex numbers are involved:
>
> import sympy
> import numpy as N
> import scipy.linalg
>
> pi=N.pi
> g=sympy.exp(2*pi*sympy.I)
> A=N.matrix([[g,0],[0,g]])
>
> scipy.linalg.eig(A)
> Traceback (most recent call last):
> File "<input>", line 1, in <module>
> File "c:\python25\lib\site-packages\scipy\linalg\decomp.py", line 149, in
> eig
> overwrite_a=overwrite_a)
> File "c:\python25\lib\site-packages\sympy\core\basic.py", line 1860, in
> __float__
> result = self.evalf()
> File "c:\python25\Lib\site-packages\sympy\core\evalf.py", line 950, in
> Basic_evalf
> prec = dps_to_prec(n)
> File "c:\python25\Lib\site-packages\sympy\mpmath\lib.py", line 102, in
> dps_to_prec
> return max(1, int(round((int(n)+1)*3.3219280948873626)))
> File "c:\python25\lib\site-packages\sympy\core\basic.py", line 1864, in
> __float__
> raise ValueError("Symbolic value, can't compute")
> ValueError: Symbolic value, can't compute
>
>
> I think that the problem is that if there is a complex number that should
> result from the evaluation of the exponential, the imaginary portion is left
> as a symbol instead of being converted to a complex number. Any ideas?
>
>
> Thanks,
> William
This does not look like a bug in evalf. It seems that numpy is calling
float(), which fails because g actually has nonzero imaginary part:
>>> complex(g)
(1-2.4286128663675299e-016j)
Is there a good reason for using numpy.pi instead of sympy.pi? With
sympy.pi, the complex exponential will simplify symbolically to a real
number.
Generally, I think using numpy linear algebra functions with sympy
elements is unlikely to work well (if at all). You should probably
convert all elements to float or complex first.
Fredrik
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