Status: New
Owner: ----
Labels: Type-Defect Priority-Medium
New issue 1420 by 2006.Uchida: subs() seems to fail against D(x(t), t)
http://code.google.com/p/sympy/issues/detail?id=1420
Dear Sympy’s developers:
I want to appreciate you all first. I'm grateful for giving us a chance to
use the feature of CAS for free. Playing with Sympy for a while, I
encountered the following unexpected behavior of Sympy, so I decided to
report it.
What I did is to apply subs(t, u) to X which contains the derivative
D(x(t), t) like the console output below.
>>> import sys; print '%s %s' % (sys.executable or sys.platform,
>>> sys.version)
C:\Program Files\Python252\python.exe 2.5.2 (r252:60911, Feb 21 2008,
13:11:45) [MSC v.1310 32 bit (Intel)]
>>> import sympy
>>> x, t, u = sympy.symbols("x t u")
>>> X = x(t).diff(t)
>>> X
D(x(t), t)
>>> X.subs(t, u)
0
In the last line, X.subs(t, u) was expected to return D(x(u), u) replacing
t to u, but it returned zero. I’m not sure whether it is a bug or my
misunderstanding of the usage of subs().
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