
C=2/3;
[z,t] = meshgrid(0:.01:3,0:.01:1.5);

%for k=4;
figure(1)
R4=64*C^3*t.^3 + 48*C^2*t.^2 - 27*C*t.*z.^2 + 12*C*t + 1;
contour(z,t,R4,[0 0],'linewidth',2)
hold on
%for k=5;
    R5 = (16875*C^4*t.^4 + 13500*C^3*t.^3 - 1280*C*t.*z.^3 + 4050*C^2*t.^2 + 540*C*t + 27);
 contour(z,t,R5,[0 0],'--','linewidth',2)   