Symsum and matrix usage error

I need to write this algorithm, but I get an error like this:

This is my code:

A=[2,4,5;6,7,8;10,12,4];
n = length(A);
syms k
for i=1:n
    for j=i+1:n
        R(i,i)=sqrt(A(i,i)-symsum(R(k,i)^2,k,1,i-1));
        R(i,j)=(A(i,j)-symsum(R(k,i)*R(k,j),k,1,i-1))/R(i,i);
         end
end

Answer 1

[Walter Roberson]

It is never possible to use a symbolic variable as a subscript.

sum(u(1:i-1,i).*u(1:i-1,k))

If you need to generalize this to multiple i values then you should look at techniques such as using tril() and matrix multiplication.