Descartes number – Wikipedia

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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 = 32⋅72⋅112⋅132⋅22021 = (3⋅1001)2 ⋅ (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,

σ(D)=(32+3+1)⋅(72+7+1)⋅(112+11+1)⋅(132+13+1)⋅(22021+1)=(13)⋅(3⋅19)⋅(7⋅19)⋅(3⋅61)⋅(22⋅1001)=32⋅7⋅13⋅192⋅61⋅(22⋅7⋅11⋅13)=2⋅(32⋅72⋅112⋅132)⋅(192⋅61)=2⋅(32⋅72⋅112⋅132)⋅22021=2D,{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 (3cdot 19)cdot (7cdot 19)cdot (3cdot 61)cdot (22cdot 1001)\&=3^{2}cdot 7cdot 13cdot 19^{2}cdot 61cdot (22cdot 7cdot 11cdot 13)=2cdot (3^{2}cdot 7^{2}cdot 11^{2}cdot 13^{2})cdot (19^{2}cdot 61)=2cdot (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 = 192 ⋅ 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,[1] 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[edit]

Banks et al. showed in 2008 that if n is a cube-free Descartes number not divisible by

3{displaystyle 3}

, then n has over a million distinct prime divisors.

Generalizations[edit]

John Voight generalized Descartes numbers to allow negative bases. He found the example

3472112192(−127)1{displaystyle 3^{4}7^{2}11^{2}19^{2}(-127)^{1}}

.[2] Subsequent work by a group at Brigham Young University found more examples similar to Voight’s example, [2] 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.[3]

See also[edit]

  1. ^ Currently, the only known almost perfect numbers are the non-negative powers of 2, whence the only known odd almost perfect number is 20 = 1.
  2. ^ a b Nadis, Steve (September 10, 2020). “Mathematicians Open a New Front on an Ancient Number Problem”. Quanta Magazine. Retrieved 3 October 2021.
  3. ^ 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[edit]

  • 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.