---
name: "claed7.f"
section: "3"
platform: "linux"
package: "lapack-doc"
version: "3.12.1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/claed7.f"
---

# claed7.f(3)

## NAME

SRC/claed7.f

## SYNOPSIS

### Functions/Subroutines

subroutine **claed7** (n, cutpnt, qsiz, tlvls, curlvl, curpbm, d, q, ldq, rho, indxq, qstore, qptr, prmptr, perm, givptr, givcol, givnum, work, rwork, iwork, info)\
 **CLAED7** used by CSTEDC. Computes the updated eigensystem of a diagonal matrix after modification by a rank-one symmetric matrix. Used when the original matrix is dense.

## Function/Subroutine Documentation

### subroutine claed7 (integer n, integer cutpnt, integer qsiz, integer tlvls, integer curlvl, integer curpbm, real, dimension( \* ) d, complex, dimension( ldq, \* ) q, integer ldq, real rho, integer, dimension( \* ) indxq, real, dimension( \* ) qstore, integer, dimension( \* ) qptr, integer, dimension( \* ) prmptr, integer, dimension( \* ) perm, integer, dimension( \* ) givptr, integer, dimension( 2, \* ) givcol, real, dimension( 2, \* ) givnum, complex, dimension( \* ) work, real, dimension( \* ) rwork, integer, dimension( \* ) iwork, integer info)

**CLAED7** used by CSTEDC. Computes the updated eigensystem of a diagonal matrix after modification by a rank-one symmetric matrix. Used when the original matrix is dense.

**Purpose:**

```
!>
!> CLAED7 computes the updated eigensystem of a diagonal
!> matrix after modification by a rank-one symmetric matrix. This
!> routine is used only for the eigenproblem which requires all
!> eigenvalues and optionally eigenvectors of a dense or banded
!> Hermitian matrix that has been reduced to tridiagonal form.
!>
!>   T = Q(in) ( D(in) + RHO * Z*Z**H ) Q**H(in) = Q(out) * D(out) * Q**H(out)
!>
!>   where Z = Q**Hu, u is a vector of length N with ones in the
!>   CUTPNT and CUTPNT + 1 th elements and zeros elsewhere.
!>
!>    The eigenvectors of the original matrix are stored in Q, and the
!>    eigenvalues are in D.  The algorithm consists of three stages:
!>
!>       The first stage consists of deflating the size of the problem
!>       when there are multiple eigenvalues or if there is a zero in
!>       the Z vector.  For each such occurrence the dimension of the
!>       secular equation problem is reduced by one.  This stage is
!>       performed by the routine SLAED2.
!>
!>       The second stage consists of calculating the updated
!>       eigenvalues. This is done by finding the roots of the secular
!>       equation via the routine SLAED4 (as called by SLAED3).
!>       This routine also calculates the eigenvectors of the current
!>       problem.
!>
!>       The final stage consists of computing the updated eigenvectors
!>       directly using the updated eigenvalues.  The eigenvectors for
!>       the current problem are multiplied with the eigenvectors from
!>       the overall problem.
!>
```

**Parameters**

*N*

```
!>          N is INTEGER
!>         The dimension of the symmetric tridiagonal matrix.  N >= 0.
!>
```

*CUTPNT*

```
!>          CUTPNT is INTEGER
!>         Contains the location of the last eigenvalue in the leading
!>         sub-matrix.  min(1,N) <= CUTPNT <= N.
!>
```

*QSIZ*

```
!>          QSIZ is INTEGER
!>         The dimension of the unitary matrix used to reduce
!>         the full matrix to tridiagonal form.  QSIZ >= N.
!>
```

*TLVLS*

```
!>          TLVLS is INTEGER
!>         The total number of merging levels in the overall divide and
!>         conquer tree.
!>
```

*CURLVL*

```
!>          CURLVL is INTEGER
!>         The current level in the overall merge routine,
!>         0 <= curlvl <= tlvls.
!>
```

*CURPBM*

```
!>          CURPBM is INTEGER
!>         The current problem in the current level in the overall
!>         merge routine (counting from upper left to lower right).
!>
```

*D*

```
!>          D is REAL array, dimension (N)
!>         On entry, the eigenvalues of the rank-1-perturbed matrix.
!>         On exit, the eigenvalues of the repaired matrix.
!>
```

*Q*

```
!>          Q is COMPLEX array, dimension (LDQ,N)
!>         On entry, the eigenvectors of the rank-1-perturbed matrix.
!>         On exit, the eigenvectors of the repaired tridiagonal matrix.
!>
```

*LDQ*

```
!>          LDQ is INTEGER
!>         The leading dimension of the array Q.  LDQ >= max(1,N).
!>
```

*RHO*

```
!>          RHO is REAL
!>         Contains the subdiagonal element used to create the rank-1
!>         modification.
!>
```

*INDXQ*

```
!>          INDXQ is INTEGER array, dimension (N)
!>         This contains the permutation which will reintegrate the
!>         subproblem just solved back into sorted order,
!>         ie. D( INDXQ( I = 1, N ) ) will be in ascending order.
!>
```

*IWORK*

```
!>          IWORK is INTEGER array, dimension (4*N)
!>
```

*RWORK*

```
!>          RWORK is REAL array,
!>                                 dimension (3*N+2*QSIZ*N)
!>
```

*WORK*

```
!>          WORK is COMPLEX array, dimension (QSIZ*N)
!>
```

*QSTORE*

```
!>          QSTORE is REAL array, dimension (N**2+1)
!>         Stores eigenvectors of submatrices encountered during
!>         divide and conquer, packed together. QPTR points to
!>         beginning of the submatrices.
!>
```

*QPTR*

```
!>          QPTR is INTEGER array, dimension (N+2)
!>         List of indices pointing to beginning of submatrices stored
!>         in QSTORE. The submatrices are numbered starting at the
!>         bottom left of the divide and conquer tree, from left to
!>         right and bottom to top.
!>
```

*PRMPTR*

```
!>          PRMPTR is INTEGER array, dimension (N lg N)
!>         Contains a list of pointers which indicate where in PERM a
!>         level's permutation is stored.  PRMPTR(i+1) - PRMPTR(i)
!>         indicates the size of the permutation and also the size of
!>         the full, non-deflated problem.
!>
```

*PERM*

```
!>          PERM is INTEGER array, dimension (N lg N)
!>         Contains the permutations (from deflation and sorting) to be
!>         applied to each eigenblock.
!>
```

*GIVPTR*

```
!>          GIVPTR is INTEGER array, dimension (N lg N)
!>         Contains a list of pointers which indicate where in GIVCOL a
!>         level's Givens rotations are stored.  GIVPTR(i+1) - GIVPTR(i)
!>         indicates the number of Givens rotations.
!>
```

*GIVCOL*

```
!>          GIVCOL is INTEGER array, dimension (2, N lg N)
!>         Each pair of numbers indicates a pair of columns to take place
!>         in a Givens rotation.
!>
```

*GIVNUM*

```
!>          GIVNUM is REAL array, dimension (2, N lg 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.
!>          > 0:  if INFO = 1, an eigenvalue did not converge
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **245** of file **claed7.f**.

## Author

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