i got two groups of numbers "a" and "b"
what is the meaning of a\b and b\a
Do you mean "groups" in the algebraic sense or just sets of numbers. Typically, if a and b are sets "a\b" means all members of a that are NOT in b. It is the same a intersect (complement b).
When you are dealing specifically with sets of numbers you should be careful to distinguish that from "a- b" which means the set of all numbers that can be formed by subtracting a number in b from a number in a.
Separate names with a comma.