A weird way of writing n? More seriously, it’s an integer variable. By convention, letters from the middle of the alphabet are used for them (generally n then

A weird way of writing n? More seriously, it’s an integer variable. By convention, letters from the middle of the alphabet are used for them (generally n then k). Here, the commenter uses k because that’s what’s used in the article and that’s what’s used because n is already used to designate the class in the definition of a residue class.