◆ Zstedc()

Eigenvalues and eigenvectors of a symmetric tridiagonal matrix to which a Hermitian matrix was reduced (Divide and conquer method)

Purpose: This routine computes all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal matrix to which a Hermitian matrix was reduced using the divide and conquer method. The eigenvectors of a full or band complex Hermitian matrix can also be found if Dsytrd or Dsptrd or Dsbtrd has been used to reduce this matrix to tridiagonal form.

Parameters

[in]	Compz	= "N": Compute eigenvalues only. = "I": Compute eigenvalues and eigenvectors of the tridiagonal matrix. = "V": Compute eigenvalues and eigenvectors of the original Hermitian matrix. On entry, Z() must contain the unitary matrix used to reduce the original matrix to tridiagonal form.
[in]	N	Order of the matrix. (N >= 0) (If N = 0, returns without computation)
[in,out]	D()	Array D(LD - 1) (LD >= N) [in] The diagonal elements of the tridiagonal matrix. [out] If Info = 0, the eigenvalues in ascending order.
[in,out]	E()	Array E(LE - 1) (LE >= N - 1) [in] The (N - 1) subdiagonal elements of the tridiagonal matrix. [out] E() has been destroyed.
[in,out]	Z()	Array Z(LZ1 - 1, LZ2 - 1) (LZ1 >= N, LZ2 >= N) [in] If Compz = "V", then Z() contains the unitary matrix used in the reduction to tridiagonal form. [out] If Info = 0, then if Compz = "V", Z() contains the orthonormal eigenvectors of the original Hermitian matrix, and if Compz = "I", Z() contains the orthonormal eigenvectors of the symmetric tridiagonal matrix. If Compz = "N", then Z() is not referenced.
[out]	Info	= 0: Successful exit. = -1: The argument Compz had an illegal value. (Compz <> "N", "V" nor "I") = -2: The argument N had an illegal value. (N < 0) = -3: The argument D() is invalid. = -4: The argument E() is invalid. = -5: The argument Z() is invalid. = i > 0: The algorithm has failed to compute an eigenvalue while working on the submatrix lying in rows and columns i/(N + 1) through mod(i, N + 1).