Is the set of aggregation functions easy to characterise? I mean functions like addition, multiplication, maximum, minimum, count... So basically everything thats commutative and associative: [tex] f(x,y)=f(y,x) [/tex] [tex] f(f(x,y),z)=f(x,f(y,z)) [/tex] What are other classes of such functions? I can't all map to just addition, can I? I'd have no idea how to do it with the max(x,y) function?!