Set of number Where any subset has a unique sum?

Consider a set of form $\{a,a+C,a+2C,a+3C\}$, each subset does not have a unique sum. Also, any set of the form you want has a subset of this form, if it has more than 3 elements. So, 3 elements is the biggest you can get of your form.


I think you want to talk about the existence of pairs which differ by one. Note that I could write your set as $\{3, 5, 4\}$, in which case the second element listed on the computer screen is not increased by 1.

For a set $S$, for all $x$ belonging to $S$, there exists a $y$ belonging to $S$, such that $(x-y)=c$. The doesn't work, since for $3$ there doesn't exist such a $y$. I guess maybe

"For a set $S$, for all $x$ belonging to $S$, there exists a y belonging to S, such that $|(x-y)|=1$"

I don't understand your problem otherwise.