TITLE: What does $c.c.$ mean in this proof?
QUESTION [5 upvotes]: This is a proof from Wikipedia of Moore-Penrose inverse being the optimal solution of a least squares problem, in which there is a acronym $c.c.$ occurred in some of the equations. Mind if I ask what does that represent?

REPLY [2 votes]: It means complex conjugate (of the term preceeding it), but in my opinion it is very lazy notation, for example, line 1 reads
\begin{align*}
  \|Ax-b\|^2 & = \|Az - b + Ax - Az\|^2\\
             &= \|Az - b\|^2 + (Az - b)^*(Ax - Az) + \overline{(Az - b)^*(Ax - Az)} + \|A(x - z)\|^2
\end{align*}