clear; clc;
n = 50; 
N = 1000; 
x = linspace(-3, 3, n)'; 
X = linspace(-3, 3, N)';
pix = pi*x; 
rng('default');
y = sin(pix) ./ (pix) + 0.1*x + 0.2*randn(n, 1);
x2 = x .^ 2; 
X2 = X .^ 2; 
hh = 2*0.3^2; 
lambda = 0.1;
k = exp(-(repmat(x2, 1, n) + repmat(x2', n, 1) - 2*(x*x'))/hh);
K = exp(-(repmat(X2, 1, n) + repmat(x2', N, 1) - 2*X*x')/hh);
t1 = k\y; 
F1 = K*t1; 
t2 = (k^2 + lambda*eye(n))\(k*y);   % L2範數約束 
F2 = K*t2;
plot(X, F1, 'g-', 'linewidth', 2);
hold on;
plot(X, F2, 'r--', 'linewidth', 2);
plot(x, y, 'bo', 'linewidth', 2);
axis([-2.8 2.8 -1 1.5]);
hold off
legend('LS','L2-Constrained LS');