Sorry, assumed you would google it.

Basically, let say you have a set of numbers {1, 2, 3, .., n}. And you remove all numbers -- except 1 -- in that set that have a common factor with n. example {1, 2, 3, 4, 5, 6} => {1, 5}. So {1, 5} is the reduced residue set of 6.

And lets call the product of all primess up to a certain prime as a primorial. (This is just like a factorial, except that we only multiply primes in the range. So, 6 is a primorial since it is 2 * 3.

Now just make a multiplication matrix with that r.r.s. and enter the product of the x and y mod the primorial. That matrix is what is generating those images.