load(draw);
H(w,a,K):= 1/ sqrt(1 + a**2 - 2*a*cos(w*K));
draw2d( terminal=svg, dimensions=[748,367],
        grid=true,user_preamble="set mxtics; set mytics",
        xtics_axis=false,ytics_axis=false,ip_grid=[20,20],
        xtics={["0",0], ["π",%pi],["2π",2*%pi],["3π",3*%pi],["4π",4*%pi],
            ["5π",5*%pi],["6π",6*%pi],["7π",7*%pi],["8π",8*%pi],
            ["9π",9*%pi],["10π",10*%pi]},
        color=blloadack,xlabel="ω (rad/s)",ylabel="{/=20 |}H ( e^{jω}){/=20 |}",
        xaxis_type=solid,yaxis_type=solid,line_width=1.5,yrange=[0,11],
        axis_bottom=true,axis_top=false,axis_right=false,axis_left=true,
        key="α = -0.9",color=blue,explicit(H(w,-0.9,1),w,0,10*%pi),
        key="α = -0.75",color=red,explicit(H(w,-0.75,1),w,0,10*%pi),
        key="α = -0.5",color=magenta,explicit(H(w,-0.5,1),w,0,10*%pi)
        );