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# Hi, there: I am computing the eigenvalues and eigenvectors of the following complex matrix. It looks that Sage does not give the right answer?

Here is the matrix that I use

B = matrix(CDF, [ [-1, 0, 1, 0, 0, 0, 1, I, -1 ], [0, 0, 0, -I, 1, I, 1, I, 0], [-1, -I, 1, -I, 1, 0, 1, 0, I], [0, 0, 0, 0, 1, I, 1, I, -1], [0, -I, 1, -I, 1, I, 1, I, 0], [-1, -I, 1, -I, 1, 0, 0, 0, 0], [-1, 0, 1, 0, 1, I, 1, I, -1], [0, -I, 1, -I, 1, I, 0, 0, 0], [-1, -I, 1, 0, 0, 0, 1, 0, -1]])

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## Comments

Could you add the values that you obtain and perhaps the version of Sage that you are using? What are the expected eigenvalues? In Sage version 8.1beta7 I get

sage: B.eigenvalues() [3.917412364954441 + 0.054291888902286756I, -0.9961206931117547 - 1.1919004224401037I, -0.8274431770819329 + 1.205961277873422I, -1.0849405169195765 - 0.6631886275784113I, -0.7276189211835324 + 0.3726506970413847I, -0.1167390153623094 + 0.6697900534213056I, 0.6516886324409439 + 0.23537500830921726I, 0.3580569417083653 - 0.08398467888210029I, -0.174295615444644 - 0.598995196646998*I]

Is that correct?

( 2017-10-11 01:47:59 -0500 )edit

## 1 answer

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The ring of complex floats CDF is not an exact ring:

sage: CDF.is_exact()
False


So while doing computation in floating point entries (i.e. in RDF or CDF) some errors may occur (see http://doc.sagemath.org/html/en/const...). You can change the base ring to the algebraic field QQbar:

sage: B = matrix(QQbar, [ [-1, 0, 1, 0, 0, 0, 1, I, -1 ], [0, 0, 0, -I, 1, I, 1, I, 0], [-1, -I, 1, -I, 1, 0, 1, 0, I], [0, 0, 0, 0, 1, I, 1, I, -1], [0, -I, 1, -I, 1, I, 1, I, 0], [-1, -I, 1, -I, 1, 0, 0, 0, 0], [-1, 0, 1, 0, 1, I, 1, I, -1], [0, -I, 1, -I, 1, I, 0, 0, 0], [-1, -I, 1, 0, 0, 0, 1, 0, -1]])
sage: B.eigenvalues()
[3.917412364954442? + 0.05429188890228639?*I, 0.6516886324409428? + 0.2353750083092179?*I, 0.3580569417083658? - 0.08398467888210074?*I, -0.1167390153623098? + 0.6697900534213050?*I, -0.1742956154446435? - 0.5989951966469979?*I, -0.7276189211835305? + 0.3726506970413855?*I, -0.8274431770819343? + 1.205961277873422?*I, -0.9961206931117568? - 1.191900422440107?*I, -1.084940516919576? - 0.6631886275784114?*I]


An alternative is to use an algebraic extension of QQusing NumberField:

sage: K.<a> = NumberField(x^2 + 1)
sage: K.is_exact()
True
sage: B = matrix(K, [ [-1, 0, 1, 0, 0, 0, 1, a, -1 ], [0, 0, 0, -a, 1, a, 1, a, 0], [-1, -a, 1, -a, 1, 0, 1, 0, a], [0, 0, 0, 0, 1, a, 1, a, -1], [0, -a, 1, -a, 1, a, 1, a, 0], [-1,-a, 1, -a, 1, 0, 0, 0, 0], [-1, 0, 1, 0, 1, a, 1, a, -1], [0, -a, 1, -a, 1, a, 0, 0, 0], [-1, -a, 1, 0, 0, 0, 1, 0, -1]])
sage: B.eigenvalues()
[-1.084940516919576? - 0.6631886275784114?*I, -0.9961206931117568? - 1.191900422440107?*I, -0.8274431770819343? + 1.205961277873422?*I, -0.7276189211835305? + 0.3726506970413855?*I, -0.1742956154446435? - 0.5989951966469979?*I, -0.1167390153623098? + 0.6697900534213050?*I, 0.3580569417083658? - 0.08398467888210074?*I, 0.6516886324409428? + 0.2353750083092179?*I, 3.917412364954442? + 0.05429188890228639?*I]

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## Comments

are there cases where one would prefer QQbar over NumberField, or vice-versa?

( 2017-10-11 03:25:26 -0500 )edit

QQbar is slower but more flexible (e.g. you can take square roots).

( 2017-10-12 01:49:30 -0500 )edit

ok, merci :)

( 2017-10-12 03:19:57 -0500 )edit

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Asked: 2017-10-10 14:17:11 -0500

Seen: 55 times

Last updated: Oct 11 '17