SRC/zlaed8.f(3)

💖 bestie, this is the SRC/zlaed8.f(3) page for zlaed8 — lowkey iconic LAPACK lore 💖

🌸 NAME 🌸

SRC/zlaed8.f — the file, the moment, the legend.

✨ SYNOPSIS ✨

🎀 Functions/Subroutines 🎀

• zlaed8 — the subroutine, bestie.

       subroutine zlaed8 (k, n, qsiz, q, ldq, d, rho, cutpnt, z, dlambda, q2,
           ldq2, w, indxp, indx, indxq, perm, givptr, givcol, givnum, info)

ZLAED8 used by ZSTEDC. Merges eigenvalues and deflates secular equation. Used when the original matrix is dense.

🫶 Function/Subroutine Documentation 🫶

🌟 subroutine zlaed8 🌟

   subroutine zlaed8 (integer k, integer n, integer qsiz, complex*16,
       dimension( ldq, * ) q, integer ldq, double precision, dimension( * ) d,
       double precision rho, integer cutpnt, double precision, dimension( * )
       z, double precision, dimension( * ) dlambda, complex*16, dimension(
       ldq2, * ) q2, integer ldq2, double precision, dimension( * ) w, integer,
       dimension( * ) indxp, integer, dimension( * ) indx, integer, dimension(
       * ) indxq, integer, dimension( * ) perm, integer givptr, integer,
       dimension( 2, * ) givcol, double precision, dimension( 2, * ) givnum,
       integer info)

ZLAED8 used by ZSTEDC. Merges eigenvalues and deflates secular equation. Used when the original matrix is dense.

💅 Purpose 💅

Okay so ZLAED8 merges the two sets of eigenvalues together into a single sorted set. Then it tries to deflate the size of the problem. There are two ways in which deflation can occur: when two or more eigenvalues are close together or if there is a tiny element in the Z vector. For each such occurrence the order of the related secular equation problem is reduced by one.

🎀 Parameters 🎀

• K — K is INTEGER. Contains the number of non-deflated eigenvalues. This is the order of the related secular equation.
• N — N is INTEGER. The dimension of the symmetric tridiagonal matrix. N >= 0.
• QSIZ — QSIZ is INTEGER. The dimension of the unitary matrix used to reduce the dense or band matrix to tridiagonal form. QSIZ >= N if ICOMPQ = 1.
• Q — Q is COMPLEX*16 array, dimension (LDQ,N). On entry, Q contains the eigenvectors of the partially solved system which has been previously updated in matrix multiplies with other partially solved eigensystems. On exit, Q contains the trailing (N-K) updated eigenvectors (those which were deflated) in its last N-K columns.
• LDQ — LDQ is INTEGER. The leading dimension of the array Q. LDQ >= max( 1, N ).
• D — D is DOUBLE PRECISION array, dimension (N). On entry, D contains the eigenvalues of the two submatrices to be combined. On exit, D contains the trailing (N-K) updated eigenvalues (those which were deflated) sorted into increasing order.
• RHO — RHO is DOUBLE PRECISION. Contains the off diagonal element associated with the rank-1 cut which originally split the two submatrices which are now being recombined. RHO is modified during the computation to the value required by DLAED3.
• CUTPNT — CUTPNT is INTEGER. Contains the location of the last eigenvalue in the leading sub-matrix. MIN(1,N) <= CUTPNT <= N.
• Z — Z is DOUBLE PRECISION array, dimension (N). On input this vector contains the updating vector (the last row of the first sub-eigenvector matrix and the first row of the second sub-eigenvector matrix). The contents of Z are destroyed during the updating process.
• DLAMBDA — DLAMBDA is DOUBLE PRECISION array, dimension (N). Contains a copy of the first K eigenvalues which will be used by DLAED3 to form the secular equation.
• Q2 — Q2 is COMPLEX*16 array, dimension (LDQ2,N). If ICOMPQ = 0, Q2 is not referenced. Otherwise, Contains a copy of the first K eigenvectors which will be used by DLAED7 in a matrix multiply (DGEMM) to update the new eigenvectors.
• LDQ2 — LDQ2 is INTEGER. The leading dimension of the array Q2. LDQ2 >= max( 1, N ).
• W — W is DOUBLE PRECISION array, dimension (N). This will hold the first k values of the final deflation-altered z-vector and will be passed to DLAED3.
• INDXP — INDXP is INTEGER array, dimension (N). This will contain the permutation used to place deflated values of D at the end of the array. On output INDXP(1:K) points to the nondeflated D-values and INDXP(K+1:N) points to the deflated eigenvalues.
• INDX — INDX is INTEGER array, dimension (N). This will contain the permutation used to sort the contents of D into ascending order.
• INDXQ — INDXQ is INTEGER array, dimension (N). This contains the permutation which separately sorts the two sub-problems in D into ascending order. Note that elements in the second half of this permutation must first have CUTPNT added to their values in order to be accurate.
• PERM — PERM is INTEGER array, dimension (N). Contains the permutations (from deflation and sorting) to be applied to each eigenblock.
• GIVPTR — GIVPTR is INTEGER. Contains the number of Givens rotations which took place in this subproblem.
• GIVCOL — GIVCOL is INTEGER array, dimension (2, N). Each pair of numbers indicates a pair of columns to take place in a Givens rotation.
• GIVNUM — GIVNUM is DOUBLE PRECISION array, dimension (2, N). Each number indicates the S value to be used in the corresponding Givens rotation.
• INFO — INFO is INTEGER. = 0: successful exit. < 0: if INFO = -i, the i-th argument had an illegal value.

💜 Author 💜

• Univ. of Tennessee
• Univ. of California Berkeley
• Univ. of Colorado Denver
• NAG Ltd.

Definition at line 225 of file zlaed8.f.

🌈 Author 🌈

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