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function yangtiao(X,Y,x) 
n=length(X); 
m=length(x); 
for i=1:n-1 
    h(i)=X(i+1)-X(i); 
end 
for i=2:n-1 
    l(i)=h(i+1)/(h(i)+h(i+1)); 
    u(i)=1-l(i); 
    g(i)=3*(l(i)*(Y(i)-Y(i-1))/h(i-1))+u(i)*(Y(i-1)-Y(i))/(-h(i-1)); 
end 
g(1)=3*(Y(2)-Y(1))/h(1); 
g(n)=3*(Y(n)-Y(n-1))/h(n-1); 
A=zeros(n); 
for i=2:n-1 
    A(i,i)=2; 
    A(i,i-1)=l(i); 
    A(i,i+1)=u(i); 
end 
A(1,1)=2; 
A(1,2)=1; 
A(n,n-1)=1; 
A(n,n)=2;