Almost perfect number
From Wikipedia, the free encyclopedia
Divisibility-based sets of integers |
Form of factorization: |
Prime number |
Composite number |
Powerful number |
Square-free number |
Achilles number |
Constrained divisor sums: |
Perfect number |
Almost perfect number |
Quasiperfect number |
Multiply perfect number |
Hyperperfect number |
Superperfect number |
Unitary perfect number |
Semiperfect number |
Primitive semiperfect number |
Practical number |
Numbers with many divisors: |
Abundant number |
Highly abundant number |
Superabundant number |
Colossally abundant number |
Highly composite number |
Superior highly composite number |
Other: |
Deficient number |
Weird number |
Amicable number |
Friendly number |
Sociable number |
Solitary number |
Sublime number |
Harmonic divisor number |
Frugal number |
Equidigital number |
Extravagant number |
See also: |
Divisor function |
Divisor |
Prime factor |
Factorization |
In mathematics, an almost perfect number (sometimes also called slightly defective number) is a natural number n such that the sum of all divisors of n (the divisor function σ(n)) is equal to 2n - 1. The only known odd almost perfect number is 1, and the only even almost perfect numbers known are those of the form 2k for some natural number k; however, it has not been shown that all almost perfect numbers are of this form. Almost perfect numbers are also known as least deficient numbers.