Linear least-squares problem
= Linear least-squares problem
{title2=$\min_x\|Ax-b\|_2$}
A linear least-squares problem minimizes $\|Ax-b\|_2$ over $x$. If $A=QR$ has full column rank, orthogonality reduces it to the upper-triangular system formed from the leading block of $R$ and the leading entries of $Q^Tb$.