TITLE: What I am missing in this simple equation from Nesterov's paper?
QUESTION [6 upvotes]: In this paper by Prof Nesterov, First-order methods of smooth convex optimization
with inexact oracle, proof of Theorem 2, there is the following very simple equation which I think it is wrong,
\begin{align}
\Vert x_{k+1}-x^*\Vert^2 = \Vert x_{k}-x^*\Vert^2 + 2\langle B(x_{k+1}-x_k),x_{k+1}-x^*\rangle -\Vert x_{k+1}-x_k\Vert^2
\end{align} 
(They defined norm as $\Vert x\Vert^2 =\langle Bx,x\rangle$)
Obviously, it must be 
\begin{align}
\Vert x_{k+1}-x^*\Vert^2 = \Vert x_k-x^*+x_{k+1}-x_{k}\Vert^2 = \Vert x_{k}-x^*\Vert^2 + 2\langle B(x_{k+1}-x_k),x_{k}-x^*\rangle \\
+\Vert x_{k+1}-x_k\Vert^2
\end{align}
Am I missing something? 
I also, checked journal version, Mathematical Programming.

REPLY [8 votes]: The author is correct. The norm in question is induced by the real inner product $(x,y):=\langle Bx,y\rangle$ where $B$ is positive definite with respect to $\langle\cdot,\cdot\rangle$. Let $u = x_{k+1} - x^\ast$ and $v = x_{k+1}-x_k$. Then the equation is simply saying that
$$
\|u\|^2 = \|u-v\|^2 + 2(v,u) - \|v\|^2,
$$
which is just a rearrangement of terms in the cosine law
$$
\|u-v\|^2 = \|u\|^2 - 2(v,u) + \|v\|^2.
$$