gives b¼_ac3=c21, using(19).To the left of this line in the parameter plane, the point of intersection ðOð1Þ nþ1,Uð1Þ nþ1Þ lies to the right of the line Onþ1 ¼_c1=ð2c2Þ, which implies that the line(80)intersects region(f)of Fig.7. However, to the right of this line, the point of intersection ðOð1Þ nþ1,Uð1Þ nþ1Þ lies to the left of the line Onþ1 ¼_c1=ð2c2Þ, which implies that the line(80) intersects regions (a)and(b)of Fig. 7. In this case, it is possible that period 2 orbits of the map may exist, since points that approach the line ‘1ðOn,UnÞ ¼ 0 as Xn-0 map onto the plane Un+1¼0 (see Section 6.2) and some points on this plane map back onto the line(80) in the plane Xn+2¼0 which intersects the line ‘1ðOn,UnÞ ¼ 0