NAME

       claev2.f -

SYNOPSIS

   Functions/Subroutines
       subroutine claev2 (A, B, C, RT1, RT2, CS1, SN1)
           CLAEV2 computes the eigenvalues and eigenvectors of a 2-by-2 symmetric/Hermitian
           matrix.

Function/Subroutine Documentation

   subroutine claev2 (complex A, complex B, complex C, real RT1, real RT2, real CS1, complex SN1)
       CLAEV2 computes the eigenvalues and eigenvectors of a 2-by-2 symmetric/Hermitian matrix.

       Purpose:

            CLAEV2 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
                    The (1,1) element of the 2-by-2 matrix.

           B

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

           C

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

           RT1

                     RT1 is REAL
                    The eigenvalue of larger absolute value.

           RT2

                     RT2 is REAL
                    The eigenvalue of smaller absolute value.

           CS1

                     CS1 is REAL

           SN1

                     SN1 is COMPLEX
                    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.

       Date:
           September 2012

       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.

Author

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