Yeah, this is wacky. I can tell you why it's happening, though someone
who's ever used Maxima before should really think about the right fix.
Here's the issue: in sage.calculus.calculus, there's an instance of
Maxima that gets created and passed the argument 'load(simplify_sum)'.
This causes the expression 'd2' to get simplified to
(3*n+k-1)!/(6*n+2*k)!. Here's a sample session illustrating the issue:
sage: maxima("d2")
d2
sage: sage.calculus.calculus.maxima("d2")
(3*n+k-1)!/(6*n+2*k)!
sage: Maxima(init_code=['load(simplify_sum)', 'display2d:false'])("d2")
(3*n+k-1)!/(6*n+2*k)!
Maybe someone who knows Maxima better could point us in the right direction?
As far as a workaround, in sage.symbolic.expression, the
Expression._maxima_ method imports maxima from sage.calculus.calculus
(which is initialized with the simplify_sum option). We could switch
that to the maxima session in sage.interfaces.maxima, but this is just
skirting the issue -- someone probably put simplify_sum there for a
reason, and I'd like to know what it is before changing it. Or, for
that matter, why do we have multiple maxima sessions running around?
-cc
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