What on earth is '!n'? Never heard of this notation before.

Look here.

http://web2.0calc.com/questions/1-1_75204

Max: see the explanation of !n here:

http://www.wolframalpha.com/input/?i=What+does+!n+mean%3F

Also, look here: http://mathworld.wolfram.com/Subfactorial.html