A number is deficient (or defective) if the sum of its factors excluding itself is less than the number.
Aside from 8, all prime numbers are deficient.
Examples of deficient numbers:
a) 9 Factors: 1, 3
Add the factors: 1 + 3 = 4, 4 <9, 9 is a deficient number
b) 14 Factors: 1, 2, 7
Add the factors: 1 + 2 + 7 = 10, 10<14, 14 is a deficient number
c) 15 Factors: 1, 3, 5
Add the factors: 1+ 3+ 5 = 13, 13< 15, 15 is a deficient number