1 votes 1 votes If A = {1, 3,5,7,.......} Then (A, +) is semi group or not. Doubt : a +(b+c) =( a + b) +c So associativity property is present But a + b doesn't belong to A So closure property is not present. So A is semi group or not. Set Theory & Algebra self-doubt group-theory set-theory + – Overflow04 asked Oct 25, 2022 Overflow04 386 views answer comment Share Follow See 1 comment See all 1 1 comment reply afroze commented Oct 25, 2022 reply Follow Share to check associativity will have to check closure property first, so it isn’t a semigroup 2 votes 2 votes Please log in or register to add a comment.