Closure property

5.1 Outline

1. Basic concepts
   1. counting numbers
   2. natural numbers
   3. addition
2. Closure property
   1. definition
   2. closed/not closed
3. Commutative and associative properties
   1. commutative property for addition
   2. associative property for addition
   3. definition of multiplication
   4. closure for multiplication in the set of natural numbers
   5. commutative property