Chapter 1 - Section 1.2 - Algebraic Expressions and Sets of Numbers - Exercise Set - Page 16: 41

$\notin$

The set {1, 3, 5, ...} defines all positive, odd integers. The number 12 is not odd. Think about continuing the set, is the number 12 included? {1, 3, 5, 7, 9, 11, 13, ...} No. The number 12 is not included in the set.