dstt21.f3

!>
!> DSTT21 checks a decomposition of the form
!>
!>    A = U S U'
!>
!> where ' means transpose, A is symmetric tridiagonal, U is orthogonal,
!> and S is diagonal (if KBAND=0) or symmetric tridiagonal (if KBAND=1).
!> Two tests are performed:
!>
!>    RESULT(1) = | A - U S U' | / ( |A| n ulp )
!>
!>    RESULT(2) = | I - UU' | / ( n ulp )
!> 

N

!>          N is INTEGER
!>          The size of the matrix.  If it is zero, DSTT21 does nothing.
!>          It must be at least zero.
!> 

KBAND

!>          KBAND is INTEGER
!>          The bandwidth of the matrix S.  It may only be zero or one.
!>          If zero, then S is diagonal, and SE is not referenced.  If
!>          one, then S is symmetric tri-diagonal.
!> 

AD

!>          AD is DOUBLE PRECISION array, dimension (N)
!>          The diagonal of the original (unfactored) matrix A.  A is
!>          assumed to be symmetric tridiagonal.
!> 

AE

!>          AE is DOUBLE PRECISION array, dimension (N-1)
!>          The off-diagonal of the original (unfactored) matrix A.  A
!>          is assumed to be symmetric tridiagonal.  AE(1) is the (1,2)
!>          and (2,1) element, AE(2) is the (2,3) and (3,2) element, etc.
!> 

SD

!>          SD is DOUBLE PRECISION array, dimension (N)
!>          The diagonal of the (symmetric tri-) diagonal matrix S.
!> 

SE

!>          SE is DOUBLE PRECISION array, dimension (N-1)
!>          The off-diagonal of the (symmetric tri-) diagonal matrix S.
!>          Not referenced if KBSND=0.  If KBAND=1, then AE(1) is the
!>          (1,2) and (2,1) element, SE(2) is the (2,3) and (3,2)
!>          element, etc.
!> 

U

!>          U is DOUBLE PRECISION array, dimension (LDU, N)
!>          The orthogonal matrix in the decomposition.
!> 

LDU

!>          LDU is INTEGER
!>          The leading dimension of U.  LDU must be at least N.
!> 

WORK

!>          WORK is DOUBLE PRECISION array, dimension (N*(N+1))
!> 

RESULT

!>          RESULT is DOUBLE PRECISION array, dimension (2)
!>          The values computed by the two tests described above.  The
!>          values are currently limited to 1/ulp, to avoid overflow.
!>          RESULT(1) is always modified.
!> 

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