Question

The set of integers is not closed for
(1) division (3) addition
(2) multiplication (4) subtraction

Answer 1

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.