ZROTG - construct givens plane rotation
subroutine zrotg(ca,cb,c,s)
double complex ca,cb,s
double precision c
double precision norm,scale
double complex alpha
if (cdabs(ca) .ne. 0.0d0) go to 10
c = 0.0d0
s = (1.0d0,0.0d0)
ca = cb
go to 20
10 continue
scale = cdabs(ca) + cdabs(cb)
norm =
scale*dsqrt((cdabs(ca/dcmplx(scale,0.0d0)))**2
+
* (cdabs(cb/dcmplx(scale,0.0d0)))**2)
alpha = ca /cdabs(ca)
c = cdabs(ca) / norm
s = alpha * dconjg(cb) / norm
ca = alpha * norm
20 continue
return
end