Fundamentals

1.Predecessor & Successor:
 The Predecessor of 16 = (16-1)= 15.
 The Successor of 16 = (16+1)= 17.
Natural Numbers: 1,2,3,4...
Whole Numbers: 0,1,2,3,4...

Note: Zero is called the identity of addition or additive identity for whole no.
          ex: 7+0=7
               15+0=15
               10+0=10
          One is the identity for multiplication of whole no. or multiplicative identity for whole no.
          ex: 7*1=7
                15*1=15
               
 2.Factors:
    a. Factors of a number is an exact divisor of that number.
    b.Factors of a number are always less than or equal to that number whose factor is said to be found.
     ex:
      Factor of 12: 1,2,3,4,6,12.

    Perfect Number: A number for which the sum of all its factors is equal to twice the number is called a perfect number.
    ex:
          Factors of 6 = 1,2,3,6.
          Sum of factors of 6 = 1+2+3+6=12.
          Therefore 6 is the Perfect number.

Note: Important Concept while finding factors for any number, while finding the factor starts with 1,2,3.. and stops where the number gets repeating.
  ex:
    Factors of 36: (1*36),(2*18),(3*12),(4*9),(6*6) Now stop dont go further
    The factors are : 1,2,3,4,6,9,12,18,36.

3.Prime and Composite Number:
   A prime number is the number whose factors are only 1 and itself are called prime numbers.
   ex: 2=1,2
         3=1,3
         4 =1,2,4
 So, 2 and 3 are prime numbers.
    Composite numbers are those numbers having more than two factors.
    ex: 10=1,2,5,10
     
     Even No.: 2,4,6,8.
     Odd No.: 1,3,5,7.

Interesting Facts:

• 2 is the smallest Even Number and smallest prime number.
• Every prime number except 2 is odd.