An eigenvalue problem for derogatory matrices

A matrix A is called derogatory   if there is more than one Jordan submatrix associated with an eigenvalue λλ. In this paper, we are concerned with the eigenvalue problem of this type of matrices.The singularities of the resolvent of A:R(z)=(A-zI)-1A:R(z)=(A-zI)-1 are exactly the eigenvalues of A. Let us consider the Laurent series of R   expanded at λλ and denote its coefficients ck(-∞⩽k⩽∞). D≔c-2D≔c-2 is the nilpotent operator, that is, there exists the order l   of λλ such that Dl≔c-l-1=0(l⩾1). Additionally, for an arbitrary vector z  , Dl-1zDl-1z is an eigenvector of λλ. Then λλ is computed from the corresponding eigenvector Dl-1zDl-1z. In order to estimate the integral representation of DkzDkz, we apply the trapezoidal rule on the circle enclosing λλ but excluding other eigenvalues of A.It is our result that, so far as related linear equations are solved with necessary precision, the eigenvalues of derogatory matrices can be computed numerically as exactly as we want and so are corresponding (generalized) eigenvectors, too.