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In number theory, a Descartes number is an odd number which would have been an odd perfect number if one of its composite factors were prime. They are named after René Descartes who observed that the number D = 3⋅7⋅11⋅13⋅22021 = (3⋅1001) ⋅ (22⋅1001 − 1) = 198585576189 would be an odd perfect number if only 22021 were a prime number, since the sum-of-divisors function for D would satisfy, if 22021 were prime,

${\displaystyle {\begin{aligned}\sigma (D)&=(3^{2}+3+1)\cdot (7^{2}+7+1)\cdot (11^{2}+11+1)\cdot (13^{2}+13+1)\cdot (22021+1)\\&=(13)\cdot (3\cdot 19)\cdot (7\cdot 19)\cdot (3\cdot 61)\cdot (22\cdot 1001)\\&=3^{2}\cdot 7\cdot 13\cdot 19^{2}\cdot 61\cdot (22\cdot 7\cdot 11\cdot 13)\\&=2\cdot (3^{2}\cdot 7^{2}\cdot 11^{2}\cdot 13^{2})\cdot (19^{2}\cdot 61)\\&=2\cdot (3^{2}\cdot 7^{2}\cdot 11^{2}\cdot 13^{2})\cdot 22021=2D,\end{aligned}}}$

where we ignore the fact that 22021 is composite (22021 = 19 ⋅ 61).

A Descartes number is defined as an odd number n = m ⋅ p where m and p are coprime and 2n = σ(m) ⋅ (p + 1), whence p is taken as a 'spoof' prime. The example given is the only one currently known.

If m is an odd almost perfect number, that is, σ(m) = 2m − 1 and 2m − 1 is taken as a 'spoof' prime, then n = m ⋅ (2m − 1) is a Descartes number, since σ(n) = σ(m ⋅ (2m − 1)) = σ(m) ⋅ 2m = (2m − 1) ⋅ 2m = 2n. If 2m − 1 were prime, n would be an odd perfect number.

Properties

Banks et al. showed in 2008 that if n is a cube-free Descartes number not divisible by ${\displaystyle 3}$, then n has over one million distinct prime divisors.

Tóth showed in 2021 that if ${\displaystyle D=pq}$ denotes a Descartes number (other than Descartes’ example), with pseudo-prime factor ${\displaystyle p}$, then ${\displaystyle q>10^{12}}$.

Generalizations

John Voight generalized Descartes numbers to allow negative bases. He found the example ${\displaystyle 3^{4}7^{2}11^{2}19^{2}(-127)^{1}}$. Subsequent work by a group at Brigham Young University found more examples similar to Voight's example, and also allowed a new class of spoofs where one is allowed to also not notice that a prime is the same as another prime in the factorization.

See also

• Erdős–Nicolas number, another type of almost-perfect number

Notes

1. Currently, the only known almost perfect numbers are the non-negative powers of 2, whence the only known odd almost perfect number is 2 = 1.
2. Banks, William D.; Güloğlu, Ahmet M.; Nevans, C. Wesley; Saidak, Filip (2008), "Descartes numbers", Anatomy of integers. Based on the CRM workshop, Montreal, Canada, March 13–17, 2006, Providence, RI: American Mathematical Society (AMS), pp. 167–173, ISBN 978-0-8218-4406-9, Zbl 1186.11004, retrieved 2024-05-13
3. ^ Nadis, Steve (September 10, 2020). "Mathematicians Open a New Front on an Ancient Number Problem". Quanta Magazine. Retrieved 3 October 2021.
4. Andersen, Nickolas; Durham, Spencer; Griffin, Michael J.; Hales, Jonathan; Jenkins, Paul; Keck, Ryan; Ko, Hankun; Molnar, Grant; Moss, Eric; Nielsen, Pace P.; Niendorf, Kyle; Tombs, Vandy; Warnick, Merrill; Wu, Dongsheng (2020). "Odd, spoof perfect factorizations". J. Number Theory (234): 31–47. arXiv:2006.10697.{{cite journal}}: CS1 maint: multiple names: authors list (link) arXiv version

References

• Banks, William D.; Güloğlu, Ahmet M.; Nevans, C. Wesley; Saidak, Filip (2008). "Descartes numbers". In De Koninck, Jean-Marie; Granville, Andrew; Luca, Florian (eds.). Anatomy of integers. Based on the CRM workshop, Montreal, Canada, March 13--17, 2006. CRM Proceedings and Lecture Notes. Vol. 46. Providence, RI: American Mathematical Society. pp. 167–173. ISBN 978-0-8218-4406-9. Zbl 1186.11004.
• Klee, Victor; Wagon, Stan (1991). Old and new unsolved problems in plane geometry and number theory. The Dolciani Mathematical Expositions. Vol. 11. Washington, DC: Mathematical Association of America. ISBN 0-88385-315-9. Zbl 0784.51002.
• Tóth, László (2021). "On the Density of Spoof Odd Perfect Numbers" (PDF). Comput. Methods Sci. Technol. 27 (1). arXiv:2101.09718..

v t e Divisibility-based sets of integers
Overview	Integer factorization Divisor Unitary divisor Divisor function Prime factor Fundamental theorem of arithmetic
Factorization forms	Prime Composite Semiprime Pronic Sphenic Square-free Powerful Perfect power Achilles Smooth Regular Rough Unusual
Constrained divisor sums	Perfect Almost perfect Quasiperfect Multiply perfect Hemiperfect Hyperperfect Superperfect Unitary perfect Semiperfect Practical Descartes Erdős–Nicolas
With many divisors	Abundant Primitive abundant Highly abundant Superabundant Colossally abundant Highly composite Superior highly composite Weird
Aliquot sequence-related	Untouchable Amicable (Triple) Sociable Betrothed
Base-dependent	Equidigital Extravagant Frugal Harshad Polydivisible Smith
Other sets	Arithmetic Deficient Friendly Solitary Sublime Harmonic divisor Refactorable Superperfect

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