Relationship between Eigenvalues and Eigenvectors of two matrices

eigenvalues-eigenvectorslinear algebramatrices

Let $T=\text{tridiag}\{-1,2,-1\}, T_1$ be almost the same matrix as $T$, but with $T_1(n,n) = 3 \ (T(n,n)-\text{ element in the n-th row and n-th column)}, and T_2$ be almost the same matrix as $T$, but with $T_2(1,1) = 3$ Find the relationship between eigenvalues and eigenvectors of $T_1$ and $T_2$.

My attempt:

Note that we can decompose $T_1$ into the sum of a diagonal matrix $D_1$ with $D_1(n,n) = 1$ and $T$. Similarly, we can write $T_2 = D_2 +T$, where $T_2(1,1) = 1.$ Since $T$ is the common term and the elements on the diagonals of $D_1$ and $D_2$ are the same, $T_1$ and $T_2$ have the same eigenvalues.

I can't figure out the connection between eigenvectors though…

Best Answer

If you read $T_1$ from the last row to the first, you get $T_2$. If $x$ is an eigenvector of $T_1$ and $y$ an eigenvector for $T_2$ for the same eigenvalue, you get $$ x_k = y_{n-k+1} $$

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