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black_bird_blue1-Visitor
1-Visitor
January 12, 2010
Question

Finger Troubles

  • January 12, 2010
  • 5 replies
  • 3210 views
O Learned Denizens,

I'm very new to MathCad but not so new to maths. I'm having trouble with understanding an answer posted to another topic, in terms of how I should actually enter the information - ie I'm struggling with the user interface a little.

What I want to do is symbolically solve for eigenvectors of a 3x3 matrix. I can do it manually with paper and pencil, and for sure it's a little cumbersome but it's not rocket science.

I can write down a 2x2 matrix of elements a11, a12, etc and persuade MatchCad to show me the symbolic solution to the eigenvalue problem.

When I ask the same thing for a 3x3 matrix I'm given the "results too large to display" message. It seems clear to me searching this forum that I need to assign the results to an array and view the results a chunk at a time.

Using an example off the forum I can break it into rows but the rows are still too large to display.

Solution:= Find(....)->
rows(Solution)->
Solution[subscript]0-> [symbolic display of interest]

Can anyone help me with the way to break down the row into further chunks so I can examine it?

Thanks in advance,

Damian

5 replies

P
ptc-13682881-Visitor
1-Visitor
January 12, 2010
On 1/12/2010 6:51:26 AM, black_bird_blue wrote:
>O Learned Denizens,
...
>What I want to do is
>symbolically solve for
>eigenvectors of a 3x3 matrix.
>...
>Thanks in advance,
>
>Damian
______________________________

For the curiosity of the matter, did you also have in mind the rotation of the matrix or nested matrix of 3D solids ? If so, there are several superb work sheets 11.2a.

jmG



T
TomGutman1-Visitor
1-Visitor
January 12, 2010
What version of Mathcad? MC14 has a very different (and in many ways more limited) symbolic processor than the earlier versions.

Are you looking for eigenvalues or eigenvectors? The two are rather different.

What is the find you show in your post (it is generally better to post actual worksheets rather than try to extract pieces as textual postings)? The usual functions for finding eigenvalues and eigenvectors are eigenvals and eigenvecs.
__________________
� � � � Tom Gutman
A
Anonymous
January 12, 2010
Damian,

Don't know what version of Mathcad you're using, but in versions 2001i through 11 the amount of "display" for symbolical results can be changed.
Refer to the Windows register change I mentioned in
http://collab.mathsoft.com/read?50632,11e#50632

It might work in versions younger than 11.

Note that increasing the value may result in displaying it, but not necessarily in that the result is actually usable.

Success!
Luc
A
AlvaroDíaz12-Amethyst
12-Amethyst
January 12, 2010
A function with the symbolic expression for eigen values for a 3x3 matrix.

Regards. Alvaro.
P
ptc-13682881-Visitor
1-Visitor
January 12, 2010
If a 3 x 3 is too large to display, what about a 19 x 19 or else huge size. The option copy to the clipboard might end up with only the internal coding but not the actual literal !!! and guess on the number of pages of either Mupad or Maple coding !!!

Decoding will be the rocket science in there, isn't ?

jmG
F
Fred_Kohlhepp23-Emerald I
23-Emerald I
January 12, 2010
It sounds to me like you're asking for a symbolic answer. Mathcad is primarily a numeric solver--if you ask it for a numeric answer to your matrix I would be very surprised if it choked. But the embedded symbolic solver (Maple before version 14, Mupad for 14) has limited use.

That said, you may still get help from some of the people who know the symbolic solvers better than I.


Fred Kohlhepp
fkohlhepp@sikorsky.com
P
PhilipOakley1-Visitor
1-Visitor
January 12, 2010
It is likely that the symbolic solver is trying too hard. In particular it is trying to cover complex values, divide by zero and other conditions in its many solutions and simplifications (it is the simplifications that can cause these special cases)

Try putting simpler values for some of the elements to see what happens.

In particular try your 2x2 example extended with just a 1 on the diagonal, and zero elsewhere. If I have guessed correctly, this should have the same basic eigen vectors and unity, (or something like that ;-), so you can then atleast check that you have a 3x3 solution for simple cases...

It is likley that the end result is that you use the numeric solution.

Or use one/many of the 'assuming' keywords to restrict the solution

Philip Oakley
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