What does this weird matrix notation mean?

Question

I came across this in a convexity chapter of a book,

$$f (\bf x) = ( \bf B x, \bf x)$$

where both $\bf B$ and $\bf x$ are matrices. What does the right-hand side mean?

Answer 1

It is $f:\mathbb R^n\to \mathbb R$ with

$$f(x)=\langle Bx,x\rangle=x^TBx$$