CSTT21  checks a decomposition of the form


    A = U S U**H


 where **H means conjugate transpose, A is real symmetric tridiagonal,


 U is unitary, and S is real and diagonal (if KBAND=0) or symmetric


 tridiagonal (if KBAND=1).  Two tests are performed:


    RESULT(1) = | A - U S U**H | / ( |A| n ulp )


    RESULT(2) = | I - U U**H | / ( n ulp )

N

          N is INTEGER


          The size of the matrix.  If it is zero, CSTT21 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 REAL array, dimension (N)


          The diagonal of the original (unfactored) matrix A.  A is


          assumed to be real symmetric tridiagonal.

AE

          AE is REAL 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 REAL array, dimension (N)


          The diagonal of the real (symmetric tri-) diagonal matrix S.

SE

          SE is REAL 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 COMPLEX array, dimension (LDU, N)


          The unitary matrix in the decomposition.

LDU

          LDU is INTEGER


          The leading dimension of U.  LDU must be at least N.

WORK

          WORK is COMPLEX array, dimension (N**2)

RWORK

          RWORK is REAL array, dimension (N)

RESULT

          RESULT is REAL 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.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.