Hello,
You're adding 'k' to the value initially stored at the location row 'lm' column 'lm' in the matrix K.
For example,
>> K=ones(3,3)
K =
1 1 1
1 1 1
1 1 1
>> k=5
k =
5
>> K(2,2)=K(2,2)+k
K =
1 1 1
1 6 1
1 1 1
