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{{Infobox number
| number = 20000
|lang1=[[Armenian numerals|Armenian]]|lang1 symbol=Ֆ}}
'''20,000''' ('''twenty thousand''') is the [[natural number]] that comes after 19,999 and before 20,001.

20,000 is a round number and is also in the title of [[Jules Verne]]'s 1870 novel ''[[Twenty Thousand Leagues Under the Seas]]''.{{Relevance inline |date=June 2024}}

==Selected numbers in the range 20001–29999==

===20001 to 20999===
* '''20002''' = number of surface-points of a tetrahedron with edge-length 100<ref>{{cite OEIS|A005893|Number of points on surface of tetrahedron}}</ref>
* '''20067''' = The smallest number with no entry in the [[Online Encyclopedia of Integer Sequences]] (OEIS)<ref>{{cite magazine |last=Bischoff |first=Manon |date=March 3, 2023 |title=The Most Boring Number in the World Is ... |url=https://www.scientificamerican.com/article/the-most-boring-number-in-the-world-is/ |magazine=Scientific American |location= |publisher=Springer Nature |access-date=}}</ref>
* '''20100''' = sum of the first 200 natural numbers (hence a [[triangular number]])
* '''20160''' = 23rd [[highly composite number]];<ref name=":0">{{Cite OEIS|A002182|Highly composite numbers}}</ref> the smallest order belonging to two non-isomorphic [[simple group]]s: the [[alternating group]] ''A''<sub>8</sub> and the [[Chevalley group]] ''A''<sub>2</sub>(4)
* '''20161''' = the largest integer that cannot be expressed as a sum of two [[abundant number]]s
* '''20230''' = [[pentagonal pyramidal number]]<ref name=":1">{{Cite OEIS|A002411|Pentagonal pyramidal numbers}}</ref>
* '''20412''' = [[Leyland number]]:<ref name=":2">{{Cite OEIS|A076980|Leyland numbers}}</ref> 9<sup>3</sup> + 3<sup>9</sup>
* '''20540''' = [[square pyramidal number]]<ref name=":3">{{Cite OEIS|A000330|Square pyramidal numbers}}</ref>
* '''20569''' = [[tetranacci number]]<ref>{{Cite OEIS|A000078|Tetranacci numbers}}</ref>
* '''20593''' = [[unique prime]] in base 12
* '''20597''' = k such that the sum of the squares of the first k primes is divisible by k.<ref>{{cite OEIS|A111441|Numbers k such that the sum of the squares of the first k primes is divisible by k|access-date=2022-06-02}}</ref>
* '''20736''' = 144<sup>2</sup> = 12<sup>4</sup>, 10000[[Duodecimal|<sub>12</sub>]], [[Palindromic number|palindromic]] in [[Positional notation|base]] 15 (6226<sub>15</sub>), also called a dozen great-gross in some [[duodecimal]] nomenclature.
* '''20793''' = [[oeis:A0001003|little Schroeder number]]
* '''20871''' = The number of weeks in exactly 400 years in the [[Gregorian calendar]]
* '''20903''' = first [[Prime number|prime]] of form 120''k'' + 23 that is ''not'' a [[full reptend prime]]

===21000 to 21999===
* '''21025''' = 145<sup>2</sup>, palindromic in base 12 (10201[[Duodecimal|<sub>12</sub>]])
* '''21147''' = [[Bell number]]<ref>{{Cite OEIS|A000110|Bell or exponential numbers}}</ref>
* '''21181''' = the least of five remaining [[Seventeen or Bust]] numbers in the [[Sierpinski number|Sierpiński problem]]
* '''21209''' = number of reduced trees with 23 nodes<ref>{{cite OEIS|A000014|Number of series-reduced trees with n nodes}}</ref>
* '''21637''' = number of partitions of 37<ref>{{cite OEIS|A000041|a(n) is the number of partitions of n (the partition numbers)}}</ref>
* '''21856''' = [[octahedral number]]<ref name=":4">{{Cite OEIS|A005900|Octahedral numbers}}</ref>
* '''21943''' = [[Friedman prime]]
* '''21952''' = 28<sup>3</sup>
* '''21978''' = reverses when multiplied by 4: 4 × 21978 = 87912

===22000 to 22999===
* '''22050''' = pentagonal pyramidal number<ref name=":1" />
* '''22140''' = square pyramidal number<ref name=":3" />
* '''22222''' = [[repdigit]], [[Kaprekar number]]:<ref>{{Cite OEIS|A006886|Kaprekar numbers}}</ref> 22222<sup>2</sup> = 493817284, 4938 + 17284 = 22222
* '''22447''' = [[cuban prime]]<ref name=":5">{{Cite OEIS|A002407|Cuban primes}}</ref>
* '''22527''' = [[Woodall number]]: 11 × 2<sup>11</sup> − 1<ref>{{Cite OEIS|A003261|Woodall numbers}}</ref>
* '''22621''' = [[repunit prime]] in [[base 12]]
* '''22699''' = one of five remaining [[Seventeen or Bust]] numbers in the [[Sierpinski number|Sierpiński problem]]

===23000 to 23999===
* '''23000''' = number of primes <math>\leq 2^{18}</math>.<ref>{{cite OEIS|A007053|Number of primes [greater than or equal to]<!--cannot figure out how to escape the less than symbol, &lt; doesn't work--> 2^n|access-date=2022-06-02}}</ref>
* '''23401''' = Leyland number:<ref name=":2" /> 6<sup>5</sup> + 5<sup>6</sup>
* '''23409''' = 153<sup>2</sup>, sum of the cubes of the first 17 positive integers
* '''23497''' = cuban prime<ref name=":5" />
* '''23821''' = square pyramidal number<ref name=":3" />
* '''23833''' = [[Padovan prime]]
* '''23969''' = octahedral number<ref name=":4" />
* '''23976''' = pentagonal pyramidal number<ref name=":1" />

===24000 to 24999===
* '''24000''' = number of primitive polynomials of degree 20 over GF(2)<ref name="ReferenceA">{{cite OEIS|A011260|Number of primitive polynomials of degree n over GF(2)}}</ref>
* '''24133''' = sum of the first 100 primes
* '''24211''' = Zeisel number<ref name=":6">{{Cite OEIS|A051015|Zeisel numbers}}</ref>
* '''24336''' = 156<sup>2</sup>, palindromic in base 5: 1234321<sub>5</sub>
* '''24389''' = 29<sup>3</sup>
* '''24571''' = cuban prime<ref name=":5" />
* '''24631''' = [[Wedderburn–Etherington prime]]<ref>{{Cite OEIS|A001190|Wedderburn-Etherington numbers}}</ref>
* '''24649''' = 157<sup>2</sup>, palindromic in base 12: 12321<sub>12</sub>
* '''24737''' = one of five remaining [[Seventeen or Bust]] numbers in the [[Sierpinski number|Sierpinski problem]]
* '''24742''' = number of signed trees with 10 nodes<ref>{{cite OEIS|A000060|Number of signed trees with n nodes}}</ref>

===25000 to 25999===
* '''25011''' = the smallest [[composite number]], ending in 1, 3, 7, or 9, that in base 10 remains composite after any insertion of a digit
* '''25085''' = Zeisel number<ref name=":6" />
* '''25117''' = cuban prime<ref name=":5" />
* '''25200''' = 224th triangular number, 24th [[highly composite number]],<ref>{{Cite web |title=A002182 - OEIS |url=https://oeis.org/A002182 |access-date=2024-11-28 |website=oeis.org}}</ref> smallest number with exactly 90 factors<ref name=":0" />
* '''25205''' = largest number whose [[factorial]] is less than 10<sup>100000</sup>
* '''25482''' = number of 21-bead necklaces (turning over is allowed) where complements are equivalent<ref>{{cite OEIS|A000011|Number of n-bead necklaces (turning over is allowed) where complements are equivalent}}</ref>
* '''25585''' = square pyramidal number<ref name=":3" />
* '''25724''' = Fine number<ref>{{cite OEIS|A000957|Fine's sequence (or Fine numbers): number of relations of valence > 0 on an n-set; also number of ordered rooted trees with n edges having root of even degree|access-date=2022-06-01}}</ref>
* '''25920''' = smallest number with exactly 70 factors

===26000 to 26999===
* '''26015''' = number of partitions of 38<ref>{{cite OEIS|A000041|a(n) is the number of partitions of n (the partition numbers)}}</ref>
* '''26214''' = octahedral number<ref name=":4" />
* '''26227''' = cuban prime<ref name=":5" />
* '''26272''' = number of 20-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed<ref>{{cite OEIS|A000013|Definition (1): Number of n-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed}}</ref>
* '''26861''' = smallest number for which there are more primes of the form 4''k'' + 1 than of the form 4''k'' + 3 up to the number, against [[Chebyshev's bias]]
* '''26896''' = 164<sup>2</sup>, palindromic in base 9: 40804<sub>9</sub>

===27000 to 27999===
* '''27000''' = 30<sup>3</sup>
* '''27405''' = [[heptagonal number]],<ref>{{cite OEIS|A000566|Heptagonal numbers}}</ref> hexadecagonal number,<ref>{{cite OEIS|A051868|Hexadecagonal numbers}}</ref> 48-gonal number, 80-gonal number, smallest integer that is polygonal in exactly 10 ways.<ref>{{cite OEIS|A063778|2=a(n) = the least integer that is polygonal in exactly n ways.}}</ref>
* '''27434''' = square pyramidal number<ref name=":3" />
* '''27559''' = Zeisel number<ref name=":6" />
* '''27594''' = number of primitive polynomials of degree 19 over GF(2)<ref name="ReferenceA"/>
* '''27648''' = 1<sup>1</sup> × 2<sup>2</sup> × 3<sup>3</sup> × 4<sup>4</sup>
* '''27653''' = Friedman prime
* '''27720''' = 25th [[highly composite number]];<ref name=":0" /> smallest number divisible by the numbers from 1 to 12 (there is no smaller number divisible by the numbers from 1 to 11 since any number divisible by 3 and 4 must be divisible by 12)
* '''27846''' = [[harmonic divisor number]]<ref>{{Cite web|url=https://oeis.org/A001599|title=Sloane's A001599 : Harmonic or Ore numbers|website=The On-Line Encyclopedia of Integer Sequences|publisher=OEIS Foundation|access-date=2016-06-15}}</ref>
* '''27889''' = 167<sup>2</sup>

===28000 to 28999===
* '''28158''' = pentagonal pyramidal number<ref name=":1" />
* '''28374''' = smallest integer to start a run of six consecutive integers with the same [[number of divisors]]
* '''28393''' = unique prime in base 13
* '''28547''' = Friedman prime
* '''28559''' = nice Friedman prime
* '''28561''' = 169<sup>2</sup> = 13<sup>4</sup> = 119<sup>2</sup> + 120<sup>2</sup>, number that is simultaneously a [[square number]] and a [[centered square number]], palindromic in base 12: 14641<sub>12</sub>
* '''28595''' = octahedral number<ref name=":4" />
* '''28657''' = [[Fibonacci prime]],<ref>{{Cite web|url=https://oeis.org/A000045|title=Sloane's A000045 : Fibonacci numbers|website=The On-Line Encyclopedia of Integer Sequences|publisher=OEIS Foundation|access-date=2016-06-15}}</ref> Markov prime<ref>{{Cite web|url=https://oeis.org/A002559|title=Sloane's A002559 : Markoff (or Markov) numbers|website=The On-Line Encyclopedia of Integer Sequences|publisher=OEIS Foundation|access-date=2016-06-15}}</ref>
* '''28900''' = 170<sup>2</sup>, palindromic in base 13: 10201<sub>13</sub>

===29000 to 29999===
* '''29241''' = 171<sup>2</sup>, sum of the cubes of the first 18 positive integers
* '''29341''' = [[Carmichael number]]<ref>{{Cite web|url=https://oeis.org/A002997|title=Sloane's A002997 : Carmichael numbers|website=The On-Line Encyclopedia of Integer Sequences|publisher=OEIS Foundation|access-date=2016-06-15}}</ref>
* '''29370''' = square pyramidal number<ref name=":3" />
* '''29527''' = Friedman prime
* '''29531''' = Friedman prime
* '''29601''' = number of planar partitions of 18<ref>{{cite OEIS|A000219|Number of planar partitions (or plane partitions) of n}}</ref>
* '''29791''' = 31<sup>3</sup>

===Primes===
There are 983 [[prime numbers]] between 20000 and 30000.

==References==
{{Reflist}}

{{Integers|10}}

[[Category:Integers|20000]]