SSTT21 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, SSTT21 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 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 (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 REAL 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 REAL array, dimension (N*(N+1))

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.

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