function []=Poisson(n)
h=1/(n+1);
A=zeros(n,n);
A(1,1)=2;
A(2,1)=-1;
A(n,n)=2;
A(n-1,n)=-1;
for i=2:n-1
    A(i,i)=2;
    A(i+1,i)=-1;
    A(i-1,i)=-1;
end
A=1/(h*h)*A;
x=h:h:1-h;
f=ones(n,1);
u=A\f;
plot([0 x 1],[0 u' 0])