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Bug introduced in 9.0 or earlier and persisting through 11.3 or later

Bug resolved in 12.0

As of 12.0, we have an unevaluated result - inconsistent with the differentiation result, but not invalid.

SeriesCoefficient[a.b[x], {x, 0, 1}] (* SeriesCoefficient[a.b[x], {x, 0, 1}] *)

If I differentiate a dot product, I get the result I expect

D[a.b[x], x] (* a.b'[x] *)

However, a series expansion of the same expression (V9-V11) gives a very different result

SeriesCoefficient[a.b[x], {x, 0, 1}] (* a.1 b'[0] *)

Is there any logical explanation of this behaviour?

EDIT

This has been confirmed as a bug by Wolfram support.

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    $\begingroup$ attempts to work with abstract vectors in mathematica are usually not fruitful. Note that second strange result can never be evaluated since Dot with a integer is not defined $\endgroup$ Commented Jun 24, 2016 at 15:03
  • $\begingroup$ bug present in 11.1.0 $\endgroup$ Commented Mar 24, 2017 at 1:11
  • $\begingroup$ I think the "edit" part should be in the answer, not the question. $\endgroup$ Commented Aug 7, 2018 at 1:52
  • $\begingroup$ I don't think this is a bug. One might regard it as a limitation that Series was not designed (or implemented) to work with Dot. But that in and of itself is not a bug. $\endgroup$ Commented Aug 7, 2018 at 13:27
  • $\begingroup$ @DanielLichtblau - If Mathematica returns an expression unevaluated, that isn't a bug. If it returns a result that I don't expect, but has a reasonable and logical explanation, that isn't a bug. Here, Mathematica has evaluated a reasonable input and returned a result that nobody has argued is logically correct. Therefore (as the email I had from Wolfram support agreed) I classify it as a bug. $\endgroup$ Commented Aug 7, 2018 at 19:21

2 Answers 2

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This has been confirmed as a bug by Wolfram support.

(Bug still present in 11.0.0)

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You can muck about with an internal function to get Series to work a bit better on Dot (and Cross etc) products. For example:

protect = Unprotect[System`Private`InternalSeries]; System`Private`InternalSeries[a_Dot, {x_, x0_, n_Integer?NonNegative}] := Module[ {d = NestList[D[#, x]&, a, n], res}, res = Quiet @ Check[d /. x->x0, $Failed]; SeriesData[x, x0, TensorExpand @ res, 0, n+1, 1] /; res =!= $Failed ] Protect @@ protect;

Now, your example works correctly:

SeriesCoefficient[a.b[x],{x,0,1}]

a.b'[0]

A more complicated example:

Series[Exp[a[x].b[x]+x^2], {x, 0, 2}] //TeXForm

$e^{a(0).b(0)}+x e^{a(0).b(0)} \left(a'(0).b(0)+a(0).b'(0)\right)+\frac{1}{2} x^2 e^{a(0).b(0)} \left(\left(a'(0).b(0)+a(0).b'(0)\right)^2+2 \left(a''(0).b(0)+2 a'(0).b'(0)+a(0).b''(0)+1\right)\right)+O\left(x^3\right)$

Where this modification won't help is when the series is not a Taylor or MacLaurin series, e.g., a Laurent or Puiseaux series.

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