Matematical notation for showing even rows of matrix

Question

I want to select an even rows of matrix. How can I show it using mathematical notations.

Something like this:

$X_{i:}$ where i is even.

Answer 1

even indices:

$X_{2i}$

uneven indices can be expressed as

$X_{2i+1}$

Of course this changes the range for $i$.

Answer 2

Let $A_{m\times n}$ be the original matrix, and $B$ be the matrix which consists of even rows of $A$. Then $B$ can be defined as follows:

$B_{i,j}=A_{2i,j}$ $\ \ (1\leq i\leq \lfloor m/2\rfloor) $