title: Find the minimum norm least squares solution of Ax = b using QR factorization when A \in R^(m×n) is not full rank
body: <p>I can only solve this when A is full rank. The solution will be R^(-1)*Q^(T)*b <br>
  And when rank(A) = r &lt; n, then the QR factorization will be A = [Q_1 Q_2] [R_1 R_2, 0 0] <br>
  I don't know how to move forward, please help.</p>