function [X, Y] = gen_nonlindata(N=2000, rad=10, thk=5, sep=5)
  setenv("GNUTERM", "x11")
  # semicirculo positivo
  p = 500;
  N = N/2;

  x = [(rad + thk) * rand(p * N, 1); - (rad + thk) * rand(p * N, 1)];
  y = [(rad + thk) * rand(2 * p * N, 1)];

  perm = randperm(2 * p * N);
  x = x(perm, :);
  y = y(perm, :);

  idx = find(sqrt(x.^2 + y.^2) <= rad + thk & sqrt(x.^2 + y.^2) >= rad);

  x = x(idx(1:N), :);
  y = y(idx(1:N), :);

  X = [ones(N, 1), x, y];
  Y = -1 * ones(N, 1);

  plot(x, y, 'ro')


  # semicirculo negativo
  x = [(rad + thk) * rand(p * N, 1); - (rad + thk) * rand(p * N, 1)];
  y = -1 * [(rad + thk) * rand(2 * p * N, 1)];

  perm = randperm(2 * p * N);
  x = x(perm, :);
  y = y(perm, :);

  idx = find(sqrt(x.^2 + y.^2) <= rad + thk & sqrt(x.^2 + y.^2) >= rad);

  x = x(idx(1:N), :) + rad + thk/2;
  y = y(idx(1:N), :) - sep;

  hold on
  plot(x, y, 'b+')

  hold off

  X = [X; ones(N, 1), x, y];
  Y = [Y; +1 * ones(N, 1)];

  perm = randperm(2*N);

  X = X(perm, :);
  Y = Y(perm, :);
end