# Math Help - Eigenvalues and Eigenvectors

1. ## Eigenvalues and Eigenvectors

Find a basis B for the domain of T such that the matix of T relative to B is diagonal.
given..
T: P1 -> P1
T(a + bx) = a + (a + 2b)x

I got confused with this hw problem. Thank you.

2. Identify the polynomial $a+bx$ with the vector $(a \ b)^t$ (transposed, can't bother texing marices ). Then T acts like multiplication by the matrix $T=
(T_1 \ T_2), \ T_1=(1 \ 1)^t , \ T_2=(0 \ 2)^t$
.

How to diagonalize this? Solve $det(T-\lambda I)=0$ for $\lambda$to get the eigenvalues 1 and 2. The corresponding eigenspaces come from $V(1)=\{a+bx: T(a+bx)=a+bx\}$ and $V(2)=\{a+bx: T(a+bx)=2(a+bx)\}$. Gladly, each space has one unit eigenvector, and they are orthogonal by theory. Arrange these in a matrix P, and you are done.