[SciPy-user] Easy way to make a block diagonal matrix?

Joseph Smidt josephsmidt@gmail....
Wed May 20 18:03:03 CDT 2009


On Wed, May 20, 2009 at 3:47 PM, Joshua Stults <joshua.stults@gmail.com> wrote:
> discretization.  Just curious, what sort of application would give you
> different matrix blocks on the diagonal?

There are several applications.  For me I need it for group theory.
In group theory, in physics at least, you can represent groups by
matrices.  If you have an NxN matrix representation of your group you
say the dimension of your representation is N.

Sometimes, you have to combine representations.  To do this you have
to combine the two representations by creating a new block diagonal
matrix of the two  representation.

For example, in standard quantum mechanics two spin 1/2 particles are
in a 3+1 state, so you represent this with a block diagonal matrix
from two matrices A which is 3x3 and B which is 1x1.  For more complex
situations you might need to create a say 6+3 representation.  In QCD
there is a 10+8+8+1 representation.

Anyways, there is an application.  Plus, you need them if you have
systems of equations where you don't want them to couple.

                          Joseph Smidt
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Joseph Smidt <josephsmidt@gmail.com>

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