slaev2.f (3) - Linux Man Pages
subroutine slaev2 (realA, realB, realC, realRT1, realRT2, realCS1, realSN1)
SLAEV2 computes the eigenvalues and eigenvectors of a 2-by-2 symmetric/Hermitian matrix.
SLAEV2 computes the eigendecomposition of a 2-by-2 symmetric matrix [ A B ] [ 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 SN1 ] [ A B ] [ CS1 -SN1 ] = [ RT1 0 ] [-SN1 CS1 ] [ B C ] [ SN1 CS1 ] [ 0 RT2 ].
A is REAL The (1,1) element of the 2-by-2 matrix.
B is REAL The (1,2) element and the conjugate of the (2,1) element of the 2-by-2 matrix.
C is REAL The (2,2) element of the 2-by-2 matrix.
RT1 is REAL The eigenvalue of larger absolute value.
RT2 is REAL The eigenvalue of smaller absolute value.
CS1 is REAL
SN1 is REAL The vector (CS1, SN1) is a unit right eigenvector for RT1.
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
- September 2012
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.
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