Updates:
Summary: subs or N has an issue with quartic/quadratic root
Comment #7 on issue 2362 by [email protected]: subs or N has an issue with
quartic/quadratic root
http://code.google.com/p/sympy/issues/detail?id=2362
Here is another example of evalf problems:
eq = x**2/(1-x)-10**(-S(10))
a,b = solve(eq)
eq.subs(x,a)
-1/10000000000 + (-1/20000000000 +
sqrt(40000000001)/20000000000)**2/(-sqrt(40000000001)/20000000000 +
20000000001/20000000000)
eq.subs(x,b).n()
0.e-133
_._prec
1 <====== that means the evaluation failed
eq.n(subs={x:b})
-1.29246970711411e-26 <==== now it worked
The evaluation shouldn't have failed when it contained all Rational
numbers. Perhaps when such a failure occurs factor should be attempted?
eq.subs(x,b)
-1/10000000000 + (-sqrt(40000000001)/20000000000 -
1/20000000000)**2/(sqrt(40000000001)/20000000000 + 20000000001/20000000000)
factor(_)
0
cancel(eq.subs(x,b))
0
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