issue with series expansion of expression with log

[Jesse Carter]

Hi -

First time poster so sorry if this has been discussed before.

I'm having an issue with doing a series expansion of an expression with a log term in it where the argument of log is close to 1.

For instance, if I do a series expansion about an arbitrary point x0, then substitute a value for x0, it works fine. But if I instead try to do an expansion about that point directly in the series function, I can get an error for certain values.

The following gives an error on the last line, but works fine if I set, say, point = 0.49.

import sympy as sym

point = 0.491

x, x0 = sym.symbols(['x', 'x0'])

f = sym.log(x/(1-x))

print(sym.series(f, x=x, x0=x0, n=1).removeO().subs(x0, point))

print(sym.series(f, x=x, x0=point, n=1).removeO())

Is the latter not an intended way of using the series expansion functionality?

For what it's worth I tried both sympy 1.4 and 1.5.1 through Anaconda.

Here is the error:

Traceback (most recent call last):

  File "sympy-bug.py", line 17, in <module>

    print(sym.series(f, x=x, x0=point,n=1).removeO())

  File "/apps/local/anaconda/Anaconda3-2019.10/lib/python3.7/site-packages/sympy/series/series.py", line 12, in series

    return expr.series(x, x0, n, dir)

  File "/apps/local/anaconda/Anaconda3-2019.10/lib/python3.7/site-packages/sympy/core/expr.py", line 2720, in series

    s = self.subs(x, rep).series(x, x0=0, n=n, dir='+', logx=logx)

  File "/apps/local/anaconda/Anaconda3-2019.10/lib/python3.7/site-packages/sympy/core/expr.py", line 2730, in series

    rv = self.subs(x, xpos).series(xpos, x0, n, dir, logx=logx)

  File "/apps/local/anaconda/Anaconda3-2019.10/lib/python3.7/site-packages/sympy/core/expr.py", line 2766, in series

    % (str(n), self))

ValueError: Could not calculate 1 terms for log((_x + 0.491)/(0.509 - _x))

Any ideas? Thanks.

   - Jesse

[Aaron Meurer]

It looks like a bug with floating point numbers. It also gives the
right answer if you instead use Rational(point). I would suggest
opening an issue for this on the SymPy issue tracker.

Aaron Meurer

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[Jesse Carter]

OK, created this issue:

https://github.com/sympy/sympy/issues/18842