+A [Deficient Number](/wiki/Deficient_Number) is a positive integer where the sum of its [Proper Divisors](/wiki/Proper_Divisor) is less than the number itself. All [Prime Number](/wiki/Prime_Number)s, for example, are deficient, their only proper divisor being 1.
+## See also
+- [Perfect Number](/wiki/Perfect_Number)
+- [Abundant Number](/wiki/Abundant_Number)
+- [Divisor Sum](/wiki/Divisor_Sum)