8 points
+ is a map from N×N to N where a + 0 = a and a + S(b) = S(a + b) (S is the successor function that gives the next number).
Then 1 + 1 = 1 + S(0) = S(1 + 0) = S(1) = 2.
4 points
9 points
No, it’s correct. You define the operation by it’s properties. It’s not saying that “a plus 0 = a” but “the result of applying the binary operation ‘+’ to any number with 0 should give the original number.”
- is just a symbol. You could instead write it as +(a,0)=a and +(a,S(b))=S(+(a,b)).
You have to have previously defined 1=S(0), 2=S(1), 3=S(2), and so on.
1 point
