The following is a worked problem:

http://img64.imageshack.us/img64/3213/34423202.gif

So, could anyone please show me how they've calculated (1,2,3)H and H(1,2,3)? I mean, for example for (1,2,3)H how did they get (1,2,3)(1,2)=(1,3)? Similarly for H(1,2,3), how (1,2)(1,2,3)=(2,3)?

Thank you.