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d = 0.8; k1 = 2; K = k1*k2*k3;

roots([2*d*T3, (4*d^2 - K)*T3^2, 2*d*T3^3])

w = 0:0.001:10;
figure; hold on; grid on
T2v = 0.1; A = polyval([T3^2*T2v, T3^2+2*d*T3*T2v, T2v+2*d*T3, 1+K], 1i*w); plot(real(A), imag(A), 'b')
T2v = 0.8; A = polyval([T3^2*T2v, T3^2+2*d*T3*T2v, T2v+2*d*T3, 1+K], 1i*w); plot(real(A), imag(A), 'r')
T2v = 5;   A = polyval([T3^2*T2v, T3^2+2*d*T3*T2v, T2v+2*d*T3, 1+K], 1i*w); plot(real(A), imag(A), 'g')
axis([-15 10 -10 15]); legend('T2=0.1','T2=0.8','T2=5')