1      LOGICAL FUNCTION SB02OV( ALPHAR, ALPHAI, BETA )
2C
3C     RELEASE 4.0, WGS COPYRIGHT 1999.
4C
5C     PURPOSE
6C
7C     To select the unstable generalized eigenvalues for solving the
8C     discrete-time algebraic Riccati equation.
9C
10C     ARGUMENTS
11C
12C     Input/Output Parameters
13C
14C     ALPHAR  (input) DOUBLE PRECISION
15C             The real part of the numerator of the current eigenvalue
16C             considered.
17C
18C     ALPHAI  (input) DOUBLE PRECISION
19C             The imaginary part of the numerator of the current
20C             eigenvalue considered.
21C
22C     BETA    (input) DOUBLE PRECISION
23C             The (real) denominator of the current eigenvalue
24C             considered.
25C
26C     METHOD
27C
28C     The function value SB02OV is set to .TRUE. for an unstable
29C     eigenvalue (i.e., with modulus greater than or equal to one) and
30C     to .FALSE., otherwise.
31C
32C     REFERENCES
33C
34C     None.
35C
36C     NUMERICAL ASPECTS
37C
38C     None.
39C
40C     CONTRIBUTOR
41C
42C     Release 3.0: V. Sima, Katholieke Univ. Leuven, Belgium, Sep. 1997.
43C     Supersedes Release 2.0 routine SB02CX by P. Van Dooren, Philips
44C     Research Laboratory, Brussels, Belgium.
45C
46C     REVISIONS
47C
48C     -
49C
50C     KEYWORDS
51C
52C     Algebraic Riccati equation, closed loop system, continuous-time
53C     system, optimal regulator, Schur form.
54C
55C     ******************************************************************
56C
57      DOUBLE PRECISION   ONE
58      PARAMETER          ( ONE = 1.0D0 )
59C     .. Scalar Arguments ..
60      DOUBLE PRECISION   ALPHAR, ALPHAI, BETA
61C     .. External Functions ..
62      DOUBLE PRECISION   DLAPY2
63      EXTERNAL           DLAPY2
64C     .. Intrinsic Functions ..
65      INTRINSIC          ABS
66C     .. Executable Statements ..
67C
68      SB02OV = DLAPY2( ALPHAR, ALPHAI ).GE.ABS( BETA )
69C
70      RETURN
71C *** Last line of SB02OV ***
72      END
73