TITLE: Underdetermined Linear Systems and the Least Squares Solution
QUESTION [5 upvotes]: I have an underdetermined linear system, with 3 equations and four unknows. I also know an initial guess for these 4 unknows. The article I am reading says: We can solve the system using the least squares method, starting form a guess. I don't know how can I do this. Thanks.

REPLY [2 votes]: @Matt L. solution is correct under the assumption $ A $ is full rank.  
If it is otherwise, the solution using the SVD is always well defined which minimizes both the norm of the error and the norm of the solution.