SRC/zlaev2.f(3)

NAME
SRC/zlaev2.f

SYNOPSIS

  Functions/Subroutines
subroutine zlaev2 (a, b, c, rt1, rt2, cs1, sn1)
ZLAEV2 computes the eigenvalues and eigenvectors of a 2-by-2 symmetric/Hermitian matrix.

Function/Subroutine Documentation

  subroutine zlaev2 (complex*16 a, complex*16 b, complex*16 c, double precision rt1, double precision rt2, double precision cs1, complex*16 sn1)

ZLAEV2 computes the eigenvalues and eigenvectors of a 2-by-2 symmetric/Hermitian matrix.

Purpose:

     ZLAEV2 computes the eigendecomposition of a 2-by-2 Hermitian matrix
        [  A         B  ]
        [  CONJG(B)  C  ].
     On return, RT1 is the eigenvalue of larger absolute value, RT2 is the
     eigenvalue of smaller absolute value, and (CS1,SN1) is the unit right
     eigenvector for RT1, giving the decomposition
     [ CS1  CONJG(SN1) ] [    A     B ] [ CS1 -CONJG(SN1) ] = [ RT1  0  ]
     [-SN1     CS1     ] [ CONJG(B) C ] [ SN1     CS1     ]   [  0  RT2 ].

Parameters
    A

              A is COMPLEX*16
             The (1,1) element of the 2-by-2 matrix.

    B

              B is COMPLEX*16
             The (1,2) element and the conjugate of the (2,1) element of
             the 2-by-2 matrix.

    C

              C is COMPLEX*16
             The (2,2) element of the 2-by-2 matrix.

    RT1

              RT1 is DOUBLE PRECISION
             The eigenvalue of larger absolute value.

    RT2

              RT2 is DOUBLE PRECISION
             The eigenvalue of smaller absolute value.

    CS1

              CS1 is DOUBLE PRECISION

    SN1

              SN1 is COMPLEX*16
             The vector (CS1, SN1) is a unit right eigenvector for RT1.

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Further Details:

      RT1 is accurate to a few ulps barring over/underflow.
      RT2 may be inaccurate if there is massive cancellation in the
      determinant A*C-B*B; higher precision or correctly rounded or
      correctly truncated arithmetic would be needed to compute RT2
      accurately in all cases.
      CS1 and SN1 are accurate to a few ulps barring over/underflow.
      Overflow is possible only if RT1 is within a factor of 5 of overflow.
      Underflow is harmless if the input data is 0 or exceeds
         underflow_threshold / macheps.

Definition at line 120 of file zlaev2.f.

Author

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