       Re: How to simplify certain covariance expressions using

• To: mathgroup at smc.vnet.net
• Subject: [mg128039] Re: How to simplify certain covariance expressions using
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Mon, 10 Sep 2012 04:06:20 -0400 (EDT)
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```You cannot run variable names together. Multiplication needs a space
or an explicit *

cov = (ea + ex) (ea + ey);

Assuming[ex*ey == 0, Simplify[cov]]

ea (ea + ex + ey)

Simplify[cov, Assumptions -> ex*ey == 0]

ea (ea + ex + ey)

Or more compactly,

Simplify[cov, ex*ey == 0]

ea (ea + ex + ey)

Or by replacement,

(cov // Expand) /. ex*ey -> 0

ea^2 + ea*ex + ea*ey

Bob Hanlon

On Sun, Sep 9, 2012 at 5:33 PM,  <hewei2004 at gmail.com> wrote:
> Hi,
>
>   I have a certain case of covariance to simplify.
>
> Suppose I have the following expression:
> cov=(ea+ex)(ea+ey)=eaea+eaex+eaey+exey
>
> How can I assume that whenever ex times ey ,the product is 0, so that the expression I want to get out of is
>
> Assuming[exey=0,Refine[cov]]=eaea+eaex+eaey
> So that the last term:exey, drops out.
>
> My problem is that the Assuming[] function won't take exey=0 as a valid assumption. Is there any way to do this using some other function or I typed something wrong?
>

```

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