Calculating number of possible combinations

Hi,

Im sure there is an easy answer to my question, but its been a while since maths class and I cant seem to work it out in my head!

Imagine I have two objects, lets call them 'A' and 'B', what I want to do is work out a formula for the total number of possible combinations of the objects, with repeats **not** allowed. so... for two letters the possibilities are:

1)A

2)B

3)A+B

3 possible combinations (B+A isnt valid as its the same as A+B as far as im concerned).

and for three letters:

1)A

2)B

3)C

4)A+B

5)A+C

6)B+C

7)A+B+C

(7 combinations)

what I would ideally like would be a formula for N objects! I understad the use of factorials (n!) in similar problems, but cant quite find the way to relate it to this... Can anyone help me?

Thanks!