I’m neither particularly excellent nor particularly poor in mathematics; however, I’m somewhat interested in practical applications of “pure” mathematics, and I’ve always had good math grades in (the German equivalent of) college, so I guess grasping this wouldn’t be totally beyond my scope.

On the net, I stumbled over this page, dealing with a check digit algorithm used in various numbering schemes. It mentions the “permutation” s = (0)(1,2,4,8,7,5)(3,6)(9) [s being the Greek letter sigma, don’t know how to code this] to be applied to the single digits of a number.

From other web sources, I’ve learned that this equals doubling the digits, and subtracting 9 if the result of the doubling is larger than 9. So could anybody please (try to) explain to me how s = (0)(1,2,4,8,7,5)(3,6)(9) is supposed to work, i.e. what the grouping of the digits in brackets means? From what I remember from statistics, permutations are the various possibilities of grouping a set of objects in different orders.