Math notation for expressions using logical (binary) masks on images (matrix)

Question

I need to write up what I have written in matlab:

I have an image (that is, a matrix) $K$ and some other matrix of the same size $M$ which acts as a mask which represents the values I would like to use in some expression.

e.g. to use a simple example:

K = magic(4)

K =

16     2     3    13
 5    11    10     8
 9     7     6    12
 4    14    15     1

M = logical([0 0 1 0; 0 1 0 0; 0 0 0 0; 0 0 0 0])

M =

 0     0     1     0
 0     1     0     0
 0     0     0     0
 0     0     0     0

max(K(M))

ans =

11

1. What is the usual standard math notation for expressing masks used in texts/journals?
2. What is the notation for the last command using argmax?
3. How would they look using a single index $i$ versus $(x,y)$ type indexes?

Hope that is clear!