Time-Dependent Degenerate Pertubation Theory for 3x3 matrix

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SUMMARY

The discussion focuses on applying Time-Dependent Degenerate Perturbation Theory to a 3x3 matrix system defined by the Hamiltonians H0 = [2,0,0;0,2,0;0,0,4] and H1 = [0,1,0;1,0,1;0,1,0]. The key challenge is diagonalizing the perturbation H1 within the 2x2 subspace corresponding to the degenerate eigenvalues of H0. The solution involves constructing a 2x2 matrix V from the inner products of the eigenvectors of H0 with H1, specifically V_{11} = ⟨u_1|H_1|u_1⟩ and V_{12} = ⟨u_1|H_1|u_2⟩, which must be diagonalized to find the energy eigenvalues to second order.

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Homework Statement


H0 = [2,0,0;0,2,0;0,0,4]
H1 = [0,1,0;1,0,1;0,1,0]

Find energy eigenvalues to 2nd order.

Homework Equations





The Attempt at a Solution


I know that I need to diagonalize the perturbation in the 2x2 subspace (for my 2 degenerate eignevalues of 2 but I'm not sure how to diagonalize my perturbation in this subspace. As far as I can tell H1 is not block diagonal so I can't separate it. What am I missing? From there I think I can do the rest of the problem. Just stuck here.
 
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Let ##|u_1\rangle## and ##|u_2\rangle## be the eigenvectors of H0 that correspond to the degenerate eigenvalue.

Form the 2x2 matrix ##V## with elements ##V_{11} = \langle u_1|H_1|u_1\rangle##, ##V_{12} = \langle u_1|H_1|u_2\rangle##, etc.

That's the 2x2 matrix that you need to diagonalize.
 

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