function X = ringdataset();
X = [];
% center ring (radius 0 + 0.3*rand)
for i=1:20
    theta = 2*pi*i/20 + 0.2*rand();
    dist = 0.4*rand();
    X = [X ; dist*cos(theta) dist*sin(theta)];
end
% first ring (radius 1 + 0.3*rand)
for i=1:80
    theta = 2*pi*i/40 + 0.2*rand();
    dist = 1+0.4*rand();
    X = [X ; dist*cos(theta) dist*sin(theta)];
end
% second ring (Radius 2 + 0.3*rand
for i=1:100
    theta = 2*pi*i/50 + 0.2*rand();
    dist = 2+0.4*rand();
    X = [X ; dist*cos(theta) dist*sin(theta)];
end
% third ring (Radius 2 + 0.3*rand)
for i=1:120
    theta = 2*pi*i/60 + 0.2*rand();
    dist = 3+0.4*rand();
    X = [X ; dist*cos(theta) dist*sin(theta)];
end
X = X';