subroutine dlae2 (A, B, C, RT1, RT2)
DLAE2 computes the eigenvalues of a 2-by-2 symmetric matrix.
DLAE2 computes the eigenvalues of a 2-by-2 symmetric matrix.
Purpose:
DLAE2 computes the eigenvalues of a 2-by-2 symmetric matrix [ A B ] [ B C ]. On return, RT1 is the eigenvalue of larger absolute value, and RT2 is the eigenvalue of smaller absolute value.
Parameters:
A is DOUBLE PRECISION The (1,1) element of the 2-by-2 matrix.
B
B is DOUBLE PRECISION The (1,2) and (2,1) elements of the 2-by-2 matrix.
C
C is DOUBLE PRECISION 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.
Author:
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
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. 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 104 of file dlae2.f.
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