Eigenvalues

Notation: 𝜆

solve det(𝐴𝜆𝐼)=0
given by numpy.linalg.eighfor diagonal matrix and numpy.linalg.eig for non.

Singular value

Notation: 𝛿

𝛿2=𝜆

Eigenvector

𝐴𝑥=𝜆𝑥

where 𝑥 is Eigenvector and 𝜆 is Eigenvalues.

Applications

Problem transformations

  • Shift, (𝐴𝜎𝐼)𝑥=(𝜆𝜎)𝑥 where 𝜎 is scalar.
  • Inversion, 𝐴1𝑥=𝑥𝜆 if 𝐴 nonsingular and 𝑥0.
  • Powers, 𝐴𝑘𝑥=𝜆𝑘𝑥.
  • Polynomial, 𝑝(𝐴)𝑥=𝑝(𝜆)𝑥 where 𝑝(𝑡) is polynomial.

Example

For 𝐴 is symmetric matrix, 𝑒𝐴 can be done by

𝑒𝐴=𝑋𝑒𝜆𝑋𝑇

where 𝑋 is Eigenvector matrix

ev, v = np.linalg.eigh(A)
expmA = v @ np.diag(np.exp(ev)) @ v^T