On Mon, Feb 10, 2014 at 02:30:53AM -0500, Patrick O'Neill wrote:
> Sorry, I misremembered: it was a limit, not a derivative.� Here is an MWE:
>
> from sympy import *
>
> x = Symbol('x',positive=True)
> z = Symbol('z')
>
> def experiment1():
> ��� # Correctly returns oo
> ��� print limit(exp(x)**z,z,oo)
>
> def experiment2():
> ��� # NotImplementedError: Result depends on the sign of sign(log(exp(x) +
> 1))
> ��� print limit((exp(x) + 1)**z,z,oo)
>
> My best guess about what is happening is that in experiment1, log(exp(x))
> is getting analytically simplified to x, but in experiment2 log(exp(x) +
> 1) is not deduced to be greater than x?
In this case, it's probably a bug in log._eval_is_positive.
This, probably, will fix it:
diff --git a/sympy/functions/elementary/exponential.py
b/sympy/functions/elementary/exponential.py
index 648934f..a5aee5a 100644
--- a/sympy/functions/elementary/exponential.py
+++ b/sympy/functions/elementary/exponential.py
@@ -674,8 +674,7 @@ def _eval_is_positive(self):
return True
if arg.is_infinitesimal:
return False
- if arg.is_Number:
- return arg > 1
+ return (arg - 1).is_positive
def _eval_is_zero(self):
# XXX This is not quite useless. Try evaluating log(0.5).is_negative
Test:
In [1]: x = Symbol('x',positive=True)
In [2]: limit(exp(x)**z,z,oo)
Out[2]: ∞
In [3]: limit((exp(x) + 1)**z,z,oo)
Out[3]: ∞
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