Let T : V → w be a linear transformation, where V is a finite dim...
Be a linear operator on a finite-dimensional vector space V. Suppose that the only eigenvalues of T are scalars λ1, λ2, ..., λk. Prove that the characteristic polynomial of T is equal to (x - λ1)(x - λ2)...(x - λk).
Proof:
Let n be the dimension of V. Since T is a linear operator on V, we can represent T by a matrix A with respect to some basis of V. Let {v1, v2, ..., vn} be a basis of V. Then we can write Tvj as a linear combination of {v1, v2, ..., vn} for each j = 1, 2, ..., n.
Suppose that λ is an eigenvalue of T and let v be an eigenvector corresponding to λ. Then we have Tv = λv. Writing v as a linear combination of {v1, v2, ..., vn}, we have v = a1v1 + a2v2 + ... + anvn for some scalars a1, a2, ..., an.
Now, applying T to both sides of the equation, we have Tv = T(a1v1 + a2v2 + ... + anvn). By linearity of T, this becomes λv = a1Tv1 + a2Tv2 + ... + anTvn.
Since {v1, v2, ..., vn} is a basis, we can express each Tvi as a linear combination of {v1, v2, ..., vn}. Let Tvi = b1v1 + b2v2 + ... + bnvn for each i = 1, 2, ..., n.
Substituting these expressions into the equation above, we have λ(a1v1 + a2v2 + ... + anvn) = a1(b1v1 + b2v2 + ... + bnvn) + a2(b1v1 + b2v2 + ... + bnvn) + ... + an(b1v1 + b2v2 + ... + bnvn).
Expanding both sides and regrouping terms, we have (λ - b1)a1v1 + (λ - b2)a2v2 + ... + (λ - bn)anvn = 0.
Since {v1, v2, ..., vn} is linearly independent, the coefficients of each vi must be zero. This gives us a system of n linear equations in the variables a1, a2, ..., an. Since v is nonzero, at least one of the ai must be nonzero.
Since λ is an eigenvalue, the system of equations above has a nonzero solution. This means that the determinant of the coefficient matrix of the system is zero. Therefore, we have (λ - b1)(λ - b2)...(λ - bn) = 0.
Since λ can be any eigenvalue of T, we have the characteristic polynomial of T as (x - λ1)(x - λ2)...(x - λk), where λ1, λ2, ..., λk are the eigenvalues of T.