How to calculate generalized eigenvectors

eigenvectors

asked 2018-04-08 20:27:31 +0100

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How to calculate generalized eigenvectors of a defective matrix? e.g.:

matrix([6,-2,-1],[3,1,-1],[2,-1,2])

Daniel

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answered 2018-04-09 01:18:09 +0100

slelievre
17684 ●22 ●160 ●349 http://carva.org/samue...

Do you mean a basis for a Jordan normal form?

Then you can use jordan_form with transformation=True.

Example using the matrix in the question:

sage: a = matrix(ZZ, 3, [[6, -2, -1], [3, 1, -1], [2, -1, 2]])
sage: m, p = a.jordan_form(transformation=True)
sage: m
[3 1 0]
[0 3 1]
[0 0 3]
sage: p
[1 3 1]
[1 3 0]
[1 2 0]

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Thank you @slelievre, that is exactly what I needed.

Now I'm trying to confirm the chain of generalized eigenvectors like this:

sage: a = matrix(ZZ, 3, [[6, -2, -1], [3, 1, -1], [2, -1, 2]])
sage: d, m = a.jordan_form(transformation=True)

sage: d

sage: m

sage: I = matrix.identity(3)
sage: zero = transpose(matrix(ZZ, [[0], [0], [0]]))
sage: x1 = m.column(0)
sage: x2 = m.column(1)
sage: x3 = m.column(2)

sage: (a-d[0,0]*I)*x1 == zero

sage: (a-d[1,1]*I)*x2 == x1

sage: (a-d[2,2]*I)*x3 == x2

Please notice the first equality is False, but it should be True, what I am doing wrong?

Daniel

danielvolinski ( 2018-04-10 18:07:44 +0100 )edit

Define zero as a vector instead of a matrix:

sage: zero = vector(ZZ, 3, [0] * 3)

Then you get True for (a-d[0,0]*I)*x1 == zero.

slelievre ( 2018-04-11 11:16:52 +0100 )edit

it works, thanks!

danielvolinski ( 2018-04-11 18:47:15 +0100 )edit

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answered 2018-04-08 21:15:40 +0100

tmonteil
27413 ●31 ●204 ●517 http://wiki.sagemath.o...

updated 2018-04-08 21:17:03 +0100

I am not sure about your exact question. Does the following help ?

sage: m = matrix([[6,-2,-1],[3,1,-1],[2,-1,2]])
sage: m.eigenvalues()
[3, 3, 3]
sage: m.eigenvectors_right()
[(3, [
  (1, 1, 1)
  ], 3)]
sage: a = m-3
sage: a
[ 3 -2 -1]
[ 3 -2 -1]
[ 2 -1 -1]
sage: a.right_kernel()
Free module of degree 3 and rank 1 over Integer Ring
Echelon basis matrix:
[1 1 1]
sage: (a^2).right_kernel()
Free module of degree 3 and rank 2 over Integer Ring
Echelon basis matrix:
[1 1 0]
[0 0 1]
sage: (a^2).right_kernel().basis()
[
(1, 1, 0),
(0, 0, 1)
]
sage: (a^3).right_kernel()
Free module of degree 3 and rank 3 over Integer Ring
Echelon basis matrix:
[1 0 0]
[0 1 0]
[0 0 1]
sage: (a^3).right_kernel().basis()
[
(1, 0, 0),
(0, 1, 0),
(0, 0, 1)
]

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answered 2018-04-08 22:00:38 +0100

Emmanuel Charpentier
7744 ●6 ●50 ●153

What's wrong with

sage:  M=matrix(SR, [[6,-2,-1],[3,1,-1],[2,-1,2]]);M
[ 6 -2 -1]
[ 3  1 -1]
[ 2 -1  2]
sage: V=M.eigenvectors_left();V
[(3, [(1, -1, 0)], 3)]
sage: V[0][1][0]*M
(3, -3, 0)
sage: V=M.eigenvectors_right();V
[(3, [(1, 1, 1)], 3)]
sage: M*(V[0][1][0])
(3, 3, 3)

?

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"In linear algebra, a generalized eigenvector of an n × n matrix A {\displaystyle A} A is a vector which satisfies certain criteria which are more relaxed than those for an (ordinary) eigenvector." Generalized_eigenvector on wikipedia

Sébastien ( 2018-04-10 10:00:32 +0100 )edit

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