01:       SUBROUTINE DROTG(DA,DB,C,S)
02: *     .. Scalar Arguments ..
03:       DOUBLE PRECISION C,DA,DB,S
04: *     ..
05: *
06: *  Purpose
07: *  =======
08: *
09: *     construct givens plane rotation.
10: *     jack dongarra, linpack, 3/11/78.
11: *
12: *
13: *     .. Local Scalars ..
14:       DOUBLE PRECISION R,ROE,SCALE,Z
15: *     ..
16: *     .. Intrinsic Functions ..
17:       INTRINSIC DABS,DSIGN,DSQRT
18: *     ..
19:       ROE = DB
20:       IF (DABS(DA).GT.DABS(DB)) ROE = DA
21:       SCALE = DABS(DA) + DABS(DB)
22:       IF (SCALE.NE.0.0d0) GO TO 10
23:       C = 1.0d0
24:       S = 0.0d0
25:       R = 0.0d0
26:       Z = 0.0d0
27:       GO TO 20
28:    10 R = SCALE*DSQRT((DA/SCALE)**2+ (DB/SCALE)**2)
29:       R = DSIGN(1.0d0,ROE)*R
30:       C = DA/R
31:       S = DB/R
32:       Z = 1.0d0
33:       IF (DABS(DA).GT.DABS(DB)) Z = S
34:       IF (DABS(DB).GE.DABS(DA) .AND. C.NE.0.0d0) Z = 1.0d0/C
35:    20 DA = R
36:       DB = Z
37:       RETURN
38:       END
39: