However, if i change the adds to sums, it produces 0's? Any ideas why that may be?

tau(x):=add(add(cos((2*Pi*b*x)/(a)),b=0..{a-1}),a=1..x)

Is bringing up the message:

Error, (in tau) unable to execute add



worked it out! Once i got rid of the {} around a-1, it worked :)

Thanks Joe Riel