---
name: "dchksb.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/dchksb.f"
---

# dchksb.f(3)

## NAME

TESTING/EIG/dchksb.f

## SYNOPSIS

### Functions/Subroutines

subroutine **dchksb** (nsizes, nn, nwdths, kk, ntypes, dotype, iseed, thresh, nounit, a, lda, sd, se, u, ldu, work, lwork, result, info)\
 **DCHKSB**

## Function/Subroutine Documentation

### subroutine dchksb (integer nsizes, integer, dimension( \* ) nn, integer nwdths, integer, dimension( \* ) kk, integer ntypes, logical, dimension( \* ) dotype, integer, dimension( 4 ) iseed, double precision thresh, integer nounit, double precision, dimension( lda, \* ) a, integer lda, double precision, dimension( \* ) sd, double precision, dimension( \* ) se, double precision, dimension( ldu, \* ) u, integer ldu, double precision, dimension( \* ) work, integer lwork, double precision, dimension( \* ) result, integer info)

**DCHKSB**

**Purpose:**

```
!>
!> DCHKSB tests the reduction of a symmetric band matrix to tridiagonal
!> form, used with the symmetric eigenvalue problem.
!>
!> DSBTRD factors a symmetric band matrix A as  U S U' , where ' means
!> transpose, S is symmetric tridiagonal, and U is orthogonal.
!> DSBTRD can use either just the lower or just the upper triangle
!> of A; DCHKSB checks both cases.
!>
!> When DCHKSB is called, a number of matrix  (), a number
!> of bandwidths (), and a number of matrix  are
!> specified.  For each size (), each bandwidth () less than or
!> equal to , and each type of matrix, one matrix will be generated
!> and used to test the symmetric banded reduction routine.  For each
!> matrix, a number of tests will be performed:
!>
!> (1)     | A - V S V' | / ( |A| n ulp )  computed by DSBTRD with
!>                                         UPLO='U'
!>
!> (2)     | I - UU' | / ( n ulp )
!>
!> (3)     | A - V S V' | / ( |A| n ulp )  computed by DSBTRD with
!>                                         UPLO='L'
!>
!> (4)     | I - UU' | / ( n ulp )
!>
!> The  are specified by an array NN(1:NSIZES); the value of
!> each element NN(j) specifies one size.
!> The  are specified by a logical array DOTYPE( 1:NTYPES );
!> if DOTYPE(j) is .TRUE., then matrix type  will be generated.
!> Currently, the list of possible types is:
!>
!> (1)  The zero matrix.
!> (2)  The identity matrix.
!>
!> (3)  A diagonal matrix with evenly spaced entries
!>      1, ..., ULP  and random signs.
!>      (ULP = (first number larger than 1) - 1 )
!> (4)  A diagonal matrix with geometrically spaced entries
!>      1, ..., ULP  and random signs.
!> (5)  A diagonal matrix with  entries 1, ULP, ..., ULP
!>      and random signs.
!>
!> (6)  Same as (4), but multiplied by SQRT( overflow threshold )
!> (7)  Same as (4), but multiplied by SQRT( underflow threshold )
!>
!> (8)  A matrix of the form  U' D U, where U is orthogonal and
!>      D has evenly spaced entries 1, ..., ULP with random signs
!>      on the diagonal.
!>
!> (9)  A matrix of the form  U' D U, where U is orthogonal and
!>      D has geometrically spaced entries 1, ..., ULP with random
!>      signs on the diagonal.
!>
!> (10) A matrix of the form  U' D U, where U is orthogonal and
!>      D has  entries 1, ULP,..., ULP with random
!>      signs on the diagonal.
!>
!> (11) Same as (8), but multiplied by SQRT( overflow threshold )
!> (12) Same as (8), but multiplied by SQRT( underflow threshold )
!>
!> (13) Symmetric matrix with random entries chosen from (-1,1).
!> (14) Same as (13), but multiplied by SQRT( overflow threshold )
!> (15) Same as (13), but multiplied by SQRT( underflow threshold )
!>
```

**Parameters**

*NSIZES*

```
!>          NSIZES is INTEGER
!>          The number of sizes of matrices to use.  If it is zero,
!>          DCHKSB does nothing.  It must be at least zero.
!>
```

*NN*

```
!>          NN is INTEGER array, dimension (NSIZES)
!>          An array containing the sizes to be used for the matrices.
!>          Zero values will be skipped.  The values must be at least
!>          zero.
!>
```

*NWDTHS*

```
!>          NWDTHS is INTEGER
!>          The number of bandwidths to use.  If it is zero,
!>          DCHKSB does nothing.  It must be at least zero.
!>
```

*KK*

```
!>          KK is INTEGER array, dimension (NWDTHS)
!>          An array containing the bandwidths to be used for the band
!>          matrices.  The values must be at least zero.
!>
```

*NTYPES*

```
!>          NTYPES is INTEGER
!>          The number of elements in DOTYPE.   If it is zero, DCHKSB
!>          does nothing.  It must be at least zero.  If it is MAXTYP+1
!>          and NSIZES is 1, then an additional type, MAXTYP+1 is
!>          defined, which is to use whatever matrix is in A.  This
!>          is only useful if DOTYPE(1:MAXTYP) is .FALSE. and
!>          DOTYPE(MAXTYP+1) is .TRUE. .
!>
```

*DOTYPE*

```
!>          DOTYPE is LOGICAL array, dimension (NTYPES)
!>          If DOTYPE(j) is .TRUE., then for each size in NN a
!>          matrix of that size and of type j will be generated.
!>          If NTYPES is smaller than the maximum number of types
!>          defined (PARAMETER MAXTYP), then types NTYPES+1 through
!>          MAXTYP will not be generated.  If NTYPES is larger
!>          than MAXTYP, DOTYPE(MAXTYP+1) through DOTYPE(NTYPES)
!>          will be ignored.
!>
```

*ISEED*

```
!>          ISEED is INTEGER array, dimension (4)
!>          On entry ISEED specifies the seed of the random number
!>          generator. The array elements should be between 0 and 4095;
!>          if not they will be reduced mod 4096.  Also, ISEED(4) must
!>          be odd.  The random number generator uses a linear
!>          congruential sequence limited to small integers, and so
!>          should produce machine independent random numbers. The
!>          values of ISEED are changed on exit, and can be used in the
!>          next call to DCHKSB to continue the same random number
!>          sequence.
!>
```

*THRESH*

```
!>          THRESH is DOUBLE PRECISION
!>          A test will count as  if the , computed as
!>          described above, exceeds THRESH.  Note that the error
!>          is scaled to be O(1), so THRESH should be a reasonably
!>          small multiple of 1, e.g., 10 or 100.  In particular,
!>          it should not depend on the precision (single vs. double)
!>          or the size of the matrix.  It must be at least zero.
!>
```

*NOUNIT*

```
!>          NOUNIT is INTEGER
!>          The FORTRAN unit number for printing out error messages
!>          (e.g., if a routine returns IINFO not equal to 0.)
!>
```

*A*

```
!>          A is DOUBLE PRECISION array, dimension
!>                            (LDA, max(NN))
!>          Used to hold the matrix whose eigenvalues are to be
!>          computed.
!>
```

*LDA*

```
!>          LDA is INTEGER
!>          The leading dimension of A.  It must be at least 2 (not 1!)
!>          and at least max( KK )+1.
!>
```

*SD*

```
!>          SD is DOUBLE PRECISION array, dimension (max(NN))
!>          Used to hold the diagonal of the tridiagonal matrix computed
!>          by DSBTRD.
!>
```

*SE*

```
!>          SE is DOUBLE PRECISION array, dimension (max(NN))
!>          Used to hold the off-diagonal of the tridiagonal matrix
!>          computed by DSBTRD.
!>
```

*U*

```
!>          U is DOUBLE PRECISION array, dimension (LDU, max(NN))
!>          Used to hold the orthogonal matrix computed by DSBTRD.
!>
```

*LDU*

```
!>          LDU is INTEGER
!>          The leading dimension of U.  It must be at least 1
!>          and at least max( NN ).
!>
```

*WORK*

```
!>          WORK is DOUBLE PRECISION array, dimension (LWORK)
!>
```

*LWORK*

```
!>          LWORK is INTEGER
!>          The number of entries in WORK.  This must be at least
!>          max( LDA+1, max(NN)+1 )*max(NN).
!>
```

*RESULT*

```
!>          RESULT is DOUBLE PRECISION array, dimension (4)
!>          The values computed by the tests described above.
!>          The values are currently limited to 1/ulp, to avoid
!>          overflow.
!>
```

*INFO*

```
!>          INFO is INTEGER
!>          If 0, then everything ran OK.
!>
!>-----------------------------------------------------------------------
!>
!>       Some Local Variables and Parameters:
!>       ---- ----- --------- --- ----------
!>       ZERO, ONE       Real 0 and 1.
!>       MAXTYP          The number of types defined.
!>       NTEST           The number of tests performed, or which can
!>                       be performed so far, for the current matrix.
!>       NTESTT          The total number of tests performed so far.
!>       NMAX            Largest value in NN.
!>       NMATS           The number of matrices generated so far.
!>       NERRS           The number of tests which have exceeded THRESH
!>                       so far.
!>       COND, IMODE     Values to be passed to the matrix generators.
!>       ANORM           Norm of A; passed to matrix generators.
!>
!>       OVFL, UNFL      Overflow and underflow thresholds.
!>       ULP, ULPINV     Finest relative precision and its inverse.
!>       RTOVFL, RTUNFL  Square roots of the previous 2 values.
!>               The following four arrays decode JTYPE:
!>       KTYPE(j)        The general type (1-10) for type .
!>       KMODE(j)        The MODE value to be passed to the matrix
!>                       generator for type .
!>       KMAGN(j)        The order of magnitude ( O(1),
!>                       O(overflow^(1/2) ), O(underflow^(1/2) )
!>
```

**Author**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **290** of file **dchksb.f**.

## Author

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