Matrix product notation My lecturer has used some notation that I've never seen before: it is a (matrix) product symbol with a left-to-right arrow over the top. Does anybody know what this means?
Thanks in advance.
Edit: It looks like this:

 A: Matrix multiplication is not commutative, so (maybe) that symbol means that you are considering the product adding the next matrix to the right:
$$
X_1 X_2 \cdots X_k.
$$
Anyway, I am just guessing, I've never seen it before!
A: The arrow notation removes ambiguity when certain procedures are represented by successive matrix multiplication. 
For example, we can find $LU$ decomposition of the matrix $A$ by running Gaussian elimination on $A$. The reduced matrix is the upper triangular matrix $U$, where the reduction is represented by left multiplication of transformation matrices $E_i$ for $i \in\{1,\ldots, n-1\}$,
$$U = \left(\overset{\curvearrowleft}{\prod_{i=1}^{n-1}} E_i\right) A = E_{n-1}\cdots E_1 A.$$
Further, it can be shown that 
$$L = \overset{\curvearrowright}{\prod_{i=1}^{n-1}} E_i^{-1} = E_1^{-1}\cdots E_{n-1}^{-1}$$
is a lower triangular matrix. Whence we obtain $A = LU$.
Not great for exposition, but fine as short hand.
A: The picture suggests that these are perhaps just two lines, which give
$$\curvearrowright \prod_{i=1}^k X_i=Y$$
if you write it in one line. Then it would mean that the equation $ \prod_{i=1}^k X_i=X_1X_2\cdots X_k=Y$ follows from another relation in the text before (i.e., $\cal{X} \curvearrowright \cal{Y})$.
