[R] Interaction versus combinations

[Alex van der Spek]

I can get the interactions between factors like this:

 > idx=c(1,3,6,9)
 > jdx=idx
 > levels(interaction(idx,jdx,lex.order=TRUE))
  [1] "1.1" "1.3" "1.6" "1.9" "3.1" "3.3" "3.6" "3.9" "6.1" "6.3" "6.6" 
"6.9"
[13] "9.1" "9.3" "9.6" "9.9"

This list contains all possible interactions. Whereas I need only the 
combinations, e.g 4 over 2 = 6 total being

"1.3 " "1.6" "1.9" "3.6" "3.9" "6.9"

I have been unable to find how to get the combinations. Of course with 
some string manipulations this can be done for this toy example. There 
should be something in R to make this easy?

Thanks in advance
Alex van der Spek