The set of integers is not closed for

(1) division (3) addition

(2) multiplication (4) subtraction

Hello,

N is closed for an opération (µ) means that

a, b ∈ N : a µ b ∈N (i suppose so)

Answer (1) ex 2/3 ∉ N

Answer (4) ex 2-3 ∉ N

∀ a, b ∈ N: a+b ∈N is true; not a answer.

∀ a, b ∈ N: a*b ∈N is true; not a answer.