Dear All: I try to list all of combination among (a,b,c,d,e,f,g,h,i,j,k).I used a<-letters[1:11] ..... k<-letters[1:11] lp<-(a,b,.....,j) expand.grip(lp) There is error message. It works when the number of letters are reduced to 5. But some of combination is just same letter in the row. Actually, I don't need that. Anyone can help on this? Thanks Xin [[alternative HTML version deleted]]