Arithmetic: Number theory and Operations with Numbers Number tree Natural Numbers : Counting numbers, viz. {1, 2, 3, 4, ……} Denoted by N: NB: 0 is NOT a Natural number Whole Numbers : Invention of 0 gave rise to set of Whole Numbers. Denoted by W. The set of Natural numbers including Zero viz. {0, 1, 2, 3, 4, ……} Integers : Invention of positives and negatives gave rise to set of Integers. Negative and Positive Natural numbers and 0 viz. {……, –3, –2, –1, 0, 1, 2, 3, ……} Properties of Integers Even Integers : Any integer that is divisible by 2 without a remainder is an even integer ; the set of even integers is {. . . -4, -2, 0, 2, 4, 6, 8, . . .} Odd integer : Integers that are when divisible by 2 leaves no remainder are odd integers ; {. . . -3, -1, 1, 3, 5, . . .} is the set of odd integers. N.B: Even numbers are represented by 2n, where n = 0, 1, 2, …… Zero is also an Even number Odd numbers are represented by 2n+1, where n = 0, 1, 2, …… Prime Number : A prime number is a positive integer that has exactly two different positive divisors, 1 and itself. For example, 2, 3, 5, 7, 11, and 13 are prime numbers, but 15 is not, since 15 has four different positive divisors, 1, 3, 5, and 15.