#4477: [with patch; needs review] Allow exp() function for PowerSeriesRing
element
to compute with valid non-zero constant term
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Reporter: sengupta | Owner: sengupta
Type: enhancement | Status: assigned
Priority: minor | Milestone: sage-3.2.1
Component: basic arithmetic | Resolution:
Keywords: |
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Comment (by dmharvey):
Code is basically good. Still running doctests. A few comments:
* Indenting on "Check for non-zero constant term" line is wrong. Please
fix.
* Shouldn't have "fixes \#4477" (what will that mean in two years)
* please refactor so that self.derivative().solve_linear_de(prec) is not
called in two different places
* "if C.parent()!=self.base_ring()" should use "is not" instead of "!="
(much more efficient)
* possibly should use "if not self[0].is_zero()" instead of "if self[0]".
I can't remember why, but maybe is_zero actually does something useful (I
vaguely remember it might have something to do with testing zero-ness in
inexact rings)
* the error message "exponential of the input does not belong to the
ring" should be more useful to the user. Perhaps "exponential of the
constant term does not lie in the coefficient ring. Consider coercing to
an appropriate larger ring", or if you're feeling ambitious, invoke the
coercion machinery to automagically find that larger ring (but don't ask
me how to do that, I have no idea)
* I think that error message is also misleading in the case that the
constant term doesn't have an exp method defined. Perhaps raise a
different error in that case, e.g. "cannot exponentiate series; exp of
constant term is not defined".
* The last doctest is misleading. The power series ring is defined over
QQ, but the doctest *implicitly* creates a power series over RR and then
exponentiates that. To demonstrate the different situation clearly, the
doctest should explicitly create the power series ring over RR.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/4477#comment:4>
Sage <http://sagemath.org/>
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