Relationship between eigenvalues of 2x2 matrices within a 4x4 matrix

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SUMMARY

The discussion focuses on the relationship between the eigenvalues of a 4x4 matrix A, structured as A = [B C; 0 D], where B, C, and D are 2x2 matrices. The characteristic equation for the 2x2 matrix B is given as λ² - trB λ + det B. The participants explore how eigenvectors of B can be extended to the 4x4 matrix A, specifically considering the vector u1' = (b1, b2, 0, 0) and its interaction with A. The conversation emphasizes the need to derive eigenvalues systematically rather than relying on assumptions.

PREREQUISITES
  • Understanding of eigenvalues and eigenvectors in linear algebra
  • Familiarity with characteristic equations of matrices
  • Knowledge of matrix block structures
  • Basic proficiency in manipulating 2x2 and 4x4 matrices
NEXT STEPS
  • Study the derivation of eigenvalues for block matrices
  • Learn about the implications of the trace and determinant in eigenvalue calculations
  • Explore the concept of extending eigenvectors to higher-dimensional matrices
  • Investigate the properties of eigenvalues in relation to matrix similarity
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Students and professionals in mathematics, particularly those studying linear algebra, matrix theory, and eigenvalue problems. This discussion is beneficial for anyone looking to deepen their understanding of matrix relationships and eigenvalue derivations.

brushman
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Homework Statement


Consider a 4 x 4 matrix A =
B C
0 D
where B, C, and D are 2 x 2 matrices. What is the relationship between the eigenvalues of A, B, C, and D?

The Attempt at a Solution


I suppose you can write A as:
b1 b2 c1 c2
b3 b4 c3 c4
0 0 d1 d2
0 0 d3 d4

and then find the eigenvalues of all of them in terms of those variables, but that seems like a lot of work that might not pay off.

I know the characteristic equation for a 2x2 can be written (as follows) but I don't know what to do with it.

λ^2 - trB λ + det BThanks.
 
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consider an eigenvector of B, u1 = (b1,b2)

now let's extend that vector to be u1' = (b1,b2,0,0). How does the extended vector act on A?
 

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