20,000

Selected numbers in the range 20001–29999

20001 to 20999

• 20081 – Motorola 68K instruction for no operation
• 20100 – sum of the first 200 natural numbers
• 20160 – highly composite number; the smallest order belonging to two non-isomorphic simple groups: the alternating group A₈ and the Chevalley group A₂
• 20161 – the largest integer that cannot be expressed as a sum of two abundant numbers
• 20230 – pentagonal pyramidal number
• 20412 – Leyland number: 9³ + 3⁹
• 20540 – square pyramidal number
• 20569 – tetranacci number
• 20593 – unique prime in base 12
• 20736 – 12⁴, 10000₁₂, palindromic in base 15
• 20903 – first prime of form 120k + 23 that is not a full reptend prime

  21000 to 21999

• 21025 – 145², palindromic in base 12
• 21147 – Bell number
• 21181 – the least of five remaining Seventeen or Bust numbers in the Sierpiński problem
• 21856 – octahedral number
• 21952 – 28³
• 21978 – reverses when multiplied by 4: 4 × 21978 = 87912

  22000 to 22999

• 22050 – pentagonal pyramidal number
• 22140 – square pyramidal number
• 22222 – repdigit, Kaprekar number: 22222² = 493817284, 4938 + 17284 = 22222
• 22447 – cuban prime
• 22527 – Woodall number: 11 × 2¹¹ − 1
• 22621 – repunit prime in base 12
• 22699 – one of five remaining Seventeen or Bust numbers in the Sierpiński problem

  23000 to 23999

• 23401 – Leyland number: 6⁵ + 5⁶
• 23409 – sum of the cubes of the first 17 positive integers
• 23497 – cuban prime
• 23821 – square pyramidal number
• 23969 – octahedral number
• 23976 – pentagonal pyramidal number

  24000 to 24999

• 24211 – Zeisel number
• 24336 – 156², palindromic in base 5: 1234321₅
• 24389 – 29³
• 24571 – cuban prime
• 24601 – Jean Valjean's prisoner number in Les Misérables
• 24631 – Wedderburn-Etherington number
• 24649 – 157², palindromic in base 12: 12321₁₂
• 24737 – one of five remaining Seventeen or Bust numbers in the Sierpinski problem

  25000 to 25999

• 25011 – the first composite number that in base 10 remains composite after any insertion of a digit
• 25085 – Zeisel number
• 25117 – cuban prime
• 25200 – highly composite number
• 25205 – largest number whose factorial is less than 10¹⁰⁰⁰⁰⁰
• 25585 – square pyramidal number

  26000 to 26999

• 26214 – octahedral number
• 26227 – cuban prime
• 26861 – smallest number for which there are more primes of the form 4k + 1 than of the form 4k + 3 up to the number, against Chebyshev's bias
• 26896 – 164², palindromic in base 9: 40804₉

  27000 to 27999

• 27000 – 30³
• 27434 – square pyramidal number
• 27559 – Zeisel number
• 27648 – 1¹ × 2² × 3³ × 4⁴
• 27720 – highly composite number; smallest number divisible by the numbers 1 to 12
• 27846 – harmonic divisor number

  28000 to 28999

• 28158 – pentagonal pyramidal number
• 28374 – smallest integer to start a run of six consecutive integers with the same number of divisors
• 28393 – unique prime in base 13
• 28561 – 13⁴, 119² + 120², number that is simultaneously a square number and a centered square number, palindromic in base 12: 14641₁₂
• 28595 – octahedral number
• 28657 – Fibonacci number, Markov number
• 28900 – 170², palindromic in base 13: 10201₁₃

  29000 to 29999

• 29241 – sum of the cubes of the first 18 positive integers
• 29341 – Carmichael number
• 29370 – square pyramidal number
• 29791 – 31³