Re: Working with arrays

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• Subject: [mg131201] Re: Working with arrays
• From: Murray Eisenberg <murray at math.umass.edu>
• Date: Mon, 17 Jun 2013 06:26:31 -0400 (EDT)
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```Just use plain old "." (period).

CenterDot has no built-in meaning, as DocumentationCenter page ref/CenterDot explains. You could give it the meaning of Dot by using the Notation package, but that would merely provide a bit of notational proximity to the usual mathematical notation at the expense of the annoyance and traps of the package.

On Jun 16, 2013, at 5:19 AM, amannucci <Anthony.J.Mannucci at jpl.nasa.gov> wrote:

> I thought I understood variables. This sequence completely mystifies me:
> Clear[lA, xtest]
> lA = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
> xtest = {4, 5, 6};
> lA\[CenterDot]xtest (* First case *)
> {{a, b}, {c, d}} . {x, y} (* Second case *)
>
> The output is:
>
> {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}\[CenterDot]{4, 5, 6}
>
> {a x + b y, c x + d y}
>
> How do I force matrix multiplication to actually occur, as in the second answer? Why does Mathematica do the matrix multiply in the second case but not the first?
>
> Thanks for any help.
>
> -Tony

---
Murray Eisenberg                                    murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower            phone 413 549-1020 (H)
University of Massachusetts                               413 545-2838 (W)
710 North Pleasant Street                         fax   413 545-1801
Amherst, MA 01003-9305

```

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