이 글은 Linear Algebra and Its Applications 책을 정리한 글입니다.
Definition
- An eigenvector of an matrix A is nonzero vector such that A = for some scalar . A scalar is called an eigenvalue of A if there is a nontrivial solution of A = ; such an is called an eigenvector corresponding to
scalar 7이 eigenvalue라면 A = 이 nontrivial solution을 가져야한다.
위 식을 전개하면 (A - 7I) = 0 이고 homogeneous equation을 통해 solution을 구해보면 다음과같다.
위 예제처럼 를 고정하여 보았지만 어떠한 든 올 수 있다. 는 eqaution 이 nontrivial solution을 갖을때 matrix의 eigenvalue이다.
이때 zero vector를 포함한 위 equation의 모든 solution set의 subspace를 eigenspace of A corresponding to 라고 한다. (null space of the matrix ).
Theorem1
- The eigenvalues of a triangular matrix are the entries on its main diagonal.
Matirx A의 eigenvalue는 equation에서 nontrivial solution을 가져야 하므로 matrix 의 diagonal entry중 적어도 하나는 zero가 되어 free variable을 가져야 한다.
그렇기에 위 eigenvalues는 이 될 수 있으며 두개가 같은 값을 가져도 된다.
Theorem2
- If are eigenvectors that correspond to distinct eigenvalues of an matrix A, then set {} is linearly independent.
If A and B are matrices, then A is similar to B if there is an invertible matrix shch that , or eqivalently, .
이태 를 similarity transformation이라한다.
Theorem 3
- If matrices A and B are similar, then they have the same characteristic polynominal and hence the same eigenvalues (with the same multiplication)
Theorem 4
- An matrix A is diagonalizable if and only if A has n linearly independent eigenvectors
- In fact, , with D a diagonal matrix, if and only if the columns of P are n linearly independent eigenvectors of A. In this case, diagonal entries of D are eigenvalues of A that correspond, respectively, to the eigenvectors in P.
Theorem5
- An matirx with n distinct eigenvalues is diagonalizable.
Theorem6
Let A be an matrix whose distinct eigenvalues are
- For , the dimension of the eigenspace for is less than or equal to the multiplicity of the eigenvalue
- dimension multiplicity
- The matrix A is diagonalizable if and only if the sum of the dimensions of the eigenspaces equals , and this happens if and only if () the characteristic polynomial factors completely into linear factors and () the dimension of the eigenspace for each equals the multiplicity of .
두 eigenvalue에 해당하는 eignespace의 dimension과 eigenvalue의 multiplicity가 같고, 모든 eigenvector의 개수가 4개이며 linearly independent하므로 A는 diagonalizable하다.
모든 eigenvector들이 linearly independent하므로 matrix P는 invertible하며 matrix P 와 D는 다음과 같이 나타낼 수 있다.
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