Question:Anxious!! How to express the part of the unstable curve with a dashline. Hope someone can help me achieve this.

Question:Anxious!! How to express the part of the unstable curve with a dashline. Hope someone can help me achieve this.

Maple

How to express the part of  unstable curve with a dashline.   Hope someone can help me achieve this.

 >
 (13)
 > combine(%, 'exp');
 (14)
 > subs(I*B(t2)=I*sigma*t2-I*C(t2),B(t2)=sigma*t2-C(t2), %);
 (15)
 > conds := combine(%, 'exp');
 (16)
 > convert(conds, 'trig');
 (17)
 > cond1 := collect(coeff(%, I, 0), [diff(A(t2), t2), cos(C(t2))]);
 (18)
 > cond2 := collect(coeff(`%%`, I, 1), [diff(A(t2), t2), sin(C(t2)), cos(C(t2))]);
 (19)
 > A(t2):= A; C(t2):=C; cond1 = 0; cond2 =0;
 (20)
 > remove(has, cond1, cos)^2+remove(has, cond2, sin)^2 = simplify(select(has, cond1, cos)^2+select(has, cond2, sin)^2, 'trig');
 (21)
 > af_eq:=%;
 (22)
 > alpha[1]:=0.0087;alpha[2]:=2.5871;f0:=f[0]
 (23)
 >
 >
 >
 >
 > ##
 >
 >
 > e:='e':    f0:='f0':   alpha[1]:='alpha[1]':   alpha[2]:='alpha[2]':
 > cond1;
 (24)
 > cond2;
 (25)

 > linalg[jacobian]([-cond2,-cond1/A],[A,C]);
 (26)
 >

 > subs(cos(C)=solve(cond1,cos(C)),sin(C)=solve(cond2,sin(C)),%);
 (27)

 > map(simplify,%):

 > linalg[charpoly](%,lambda):

 > p:=collect(%,lambda);
 (28)
 >

 > coeff(p,lambda,0);
 (29)
 >

 > stab_cond:=%;
 (30)
 >

 > af_eq;
 (31)

 > with(plots, implicitplot,implicitplot3d):
 >
 > f0 :=0.1; alpha[1]:=0.0087;alpha[2]:=2.5871;
 (32)
 >

 > p1:=implicitplot(stab_cond,sigma = -1.2 .. 2, A = 0 .. 2,  numpoints = 20000, axes = box, axesfont=[SYMBOL, 14],labels = [sigma, A], labelfont = [SYMBOL, 16],color="Green",tickmarks=[9,12],thickness=2,linestyle=dash):

 > with(plots,textplot):    with(plots,display):

 > ps1:=textplot([0.75, 0.65, "Unstable region"], 'align' = {'above', 'right'},rotation = 0.6, font=[Times,bold,12]):

 > pu1:=implicitplot(af_eq,sigma = -1.2 .. 2, A = 0 .. 2,  numpoints = 20000, axes = box, axesfont=[SYMBOL, 14],labels = [sigma, A], labelfont = [SYMBOL, 16],color="red",tickmarks=[9,12],thickness=2):
 > pp1:=display({p1,ps1,pu1});
 > Like this picture.