Re: No trace of trace!

• To: mathgroup at smc.vnet.net
• Subject: [mg15140] Re: No trace of trace!
• From: David Annetts <dannetts at laurel.ocs.mq.edu.au>
• Date: Wed, 16 Dec 1998 03:11:15 -0500
• Organization: CRCAMET/Macquarie University
• References: <754ulc\$je8@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi Peter,

>
> So I'm doing my QFT homework, and I need to know what the matrices A and
> B would look like if
>
>     Tr[A] Tr[B] = Tr[AB]
>
> were true.  (For example, it's true if A=B=1 or if A=-B=1).  I quickly
> walk over to my computer desk, sit down and power up Mathematica.  I
> enter two  matrices and type Trace[A] and got wierd output.
>
> So I go to help and find out Trace is not a matrix operation.
>
> After looking for close to 5 minutes, I've concluded that the Trace
> *matrix* operation is either not present or well hidden.
>
> Can someone tell me which one?

"You can get the trace of the matrix by explicitly constructing a sum of
the elements on its leading diagonal. "

eg Sum[ m[[i, i]], {i, 2} ] where m = {{a, b}, {c, d}}

It's in Section 3.7.7: Basic Matrix operations.

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```

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