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What does the following notation for a matrix mean: $$\operatorname{diag}(v,v)$$ (to add context, this is written to be the vacuum value of the Higgs matrix).

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This might stand for $\begin{pmatrix}v & 0 \\ 0 & v\end{pmatrix}$. I'm not familiar with the context, so I can't say for sure. –  Dylan Moreland Jan 5 '12 at 3:25
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Where did you find this notation? Is $v$ a number or a vector? –  Srivatsan Jan 5 '12 at 3:25
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I'll note that I've seen $\mathrm{diag}$ used for both diagonal and block diagonal matrices. Saying where you saw this would help a great deal... –  J. M. Jan 5 '12 at 3:41
    
I have printed notes from previous years. On searching google books with the context, I found it used here –  kuch nahi Jan 5 '12 at 13:19

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Usually, $diag(\lambda_1,\ldots,\lambda_n)$ denotes the n-dimensional matrix with those specific entries along the diagonal.

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Also, does anyone know how to link this account with my other stackexchange accounts? I am under the same name and everything for the Physics and Theoretical Physics forums. –  Chris Gerig Jan 5 '12 at 4:05
    
You can try asking in meta. IS the e-mail address that you used here the same as the e-mail address in your other SE accounts? –  J. M. Jan 5 '12 at 4:53
    
It appears you are not using the same login as you are using for Physics and Theoretical Physics. How are you getting your OpenID? It appears you are using AOL for this site. What you can do is go to your own user page, click "my logins" on the top right corner, and add the other logins which you used for Physics and TP to the list of logins. Then the SE engine should automatically associate the accounts. If you still have trouble, please e-mail team@stackexchange.com about this. –  Willie Wong Jan 5 '12 at 9:58
    
cool great thanks –  Chris Gerig Jan 5 '12 at 23:30

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