ltriangsec.m

fit programme for a serials of data

function [L,U]=ltriangsec(A)
%input A  is an N*N  nonsingular matrix
%output L is an N*N unit downtriangle matrix
%       U is an N*N uptriangle matrix
[n,n]=size(A);
L=zeros(n,n);
U=zeros(n,n);
for i=1:n
    L(i,i)=1;
    U(1,i)=A(1,i);
    L(i,1)=A(i,1)/U(1,1);
end
for k=2:n
    for i=k:n
        U(k,i)=A(k,i);
        for j=1:k-1
            U(k,i)=U(k,i)-L(k,j)*U(j,i);
        end
    end
    for i=k+1:n
       L(i,k)=A(i,k); 
       for j=1:k-1
          L(i,k)=L(i,k)-L(i,j)*U(j,k); 
       end
       L(i,k)=L(i,k)/U(k,k);
    end
end