To Whom it May Concern:
Per the "Reporting Bugs" procedure
<https://doc.sagemath.org/html/en/developer/trac.html#reporting-bugs>, I am
emailing sage-devel@ with a suspected bug. I am happy to open a TRAC
ticket, if appropriate.
I'm getting some seemingly incorrect series expansions. In particular,
adding a constant is sometimes introducing an $x^{-1}$ term instead of an
$x^0$ term. I have included below a fairly minimal repro. Sage correctly
expands `(1-sqrt(1-x))/x`. However, sage insists that adding `123`
introduces a `123*x^-1` term. Using `taylor()`, I get the expected
expansion in both cases.
This seems obviously incorrect, but I am new to sage so apologize if this
is just pilot error. I looked for existing threads on power series bugs.
The closest I found was this message
<https://groups.google.com/g/sage-devel/c/_rB24fZSHKs/m/XneaemQ4AgAJ> asserting
that the combinatorial species code has long been riddled with bugs. If
this is a bug, is taylor() believed to be less bug-riddled?
*Repro:*
{{{
sage: ((1-sqrt(1-x))/x + 0).series(x,3)
1/2 + 1/8*x + 1/16*x^2 + Order(x^3) # correct
sage: ((1-sqrt(1-x))/x + 123).series(x,3)
123*x^(-1) + 1/2 + 1/8*x + 1/16*x^2 + Order(x^3) # ???
# taylor() correctly expands both
sage: taylor((1-sqrt(1-x))/x + 0,x,0,2)
1/16*x^2 + 1/8*x + 1/2
sage: taylor((1-sqrt(1-x))/x + 123,x,0,2)
1/16*x^2 + 1/8*x + 247/2
}}}
*Version:*
│ SageMath version 9.2, Release Date: 2020-10-24 │
│ Using Python 3.8.5. Type "help()" for help. │
*OS*: macOS Catalina (10.15.7) (64-bit x86)
Best,
Thomas
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