Almost perfect number

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Short description: Class of natural number
Demonstration, with Cuisenaire rods, that the number 8 is almost perfect, and deficient.

In mathematics, an almost perfect number (sometimes also called slightly defective or least deficient number) is a natural number n such that the sum of all divisors of n (the sum-of-divisors function σ(n)) is equal to 2n − 1, the sum of all proper divisors of n, s(n) = σ(n) − n, then being equal to n − 1. The only known almost perfect numbers are powers of 2 with non-negative exponents (sequence A000079 in the OEIS). Therefore the only known odd almost perfect number is 20 = 1, and the only known even almost perfect numbers are those of the form 2k for some positive integer k; however, it has not been shown that all almost perfect numbers are of this form. It is known that an odd almost perfect number greater than 1 would have at least six prime factors.[1][2]

If m is an odd almost perfect number then m(2m − 1) is a Descartes number.[3] Moreover if a and b are positive odd integers such that [math]\displaystyle{ b+3\lt a\lt \sqrt{m/2} }[/math] and such that 4ma and 4m + b are both primes, then m(4ma)(4m + b) would be an odd weird number.[4]

See also


  1. Kishore, Masao (1978). "Odd integers N with five distinct prime factors for which 2−10−12 < σ(N)/N < 2+10−12". Mathematics of Computation 32: 303–309. doi:10.2307/2006281. ISSN 0025-5718. 
  2. Kishore, Masao (1981). "On odd perfect, quasiperfect, and odd almost perfect numbers". Mathematics of Computation 36 (154): 583–586. doi:10.2307/2007662. ISSN 0025-5718. 
  3. Banks, William D.; Güloğlu, Ahmet M.; Nevans, C. Wesley; Saidak, Filip (2008). "Descartes numbers". in De Koninck, Jean-Marie; Granville, Andrew; Luca, Florian. Anatomy of integers. Based on the CRM workshop, Montreal, Canada, March 13–17, 2006. CRM Proceedings and Lecture Notes. 46. Providence, RI: American Mathematical Society. pp. 167–173. ISBN 978-0-8218-4406-9. 
  4. Melfi, Giuseppe (2015). "On the conditional infiniteness of primitive weird numbers". Journal of Number Theory 147: 508–514. doi:10.1016/j.jnt.2014.07.024. 

Further reading

External links