20,000

• 20081 = Motorola 68K instruction for no operation (NOP)
• 20100 = sum of the first 200 natural numbers (hence a triangular number)
• 20160 = highly composite number;[1] the smallest order belonging to two non-isomorphic simple groups: the alternating group A₈ and the Chevalley group A₂(4)
• 20161 = the largest integer that cannot be expressed as a sum of two abundant numbers
• 20230 = pentagonal pyramidal number[2]
• 20412 = Leyland number:[3] 9³ + 3⁹
• 20540 = square pyramidal number[4]
• 20569 = tetranacci number[5]
• 20593 = unique prime in base 12
• 20736 = 144² = 12⁴, 10000₁₂, palindromic in base 15 (6226₁₅)
• 20903 = first prime of form 120k + 23 that is not a full reptend prime

• 21025 = 145², palindromic in base 12 (10201₁₂)
• 21147 = Bell number[6]
• 21181 = the least of five remaining Seventeen or Bust numbers in the Sierpiński problem
• 21856 = octahedral number[7]
• 21943 = Friedman prime
• 21952 = 28³
• 21978 = reverses when multiplied by 4: 4 × 21978 = 87912

• 22050 = pentagonal pyramidal number[2]
• 22140 = square pyramidal number[4]
• 22222 = repdigit, Kaprekar number:[8] 22222² = 493817284, 4938 + 17284 = 22222
• 22447 = cuban prime[9]
• 22527 = Woodall number: 11 × 2¹¹ − 1[10]
• 22621 = repunit prime in base 12
• 22699 = one of five remaining Seventeen or Bust numbers in the Sierpiński problem

• 23401 = Leyland number:[3] 6⁵ + 5⁶
• 23409 = 153², sum of the cubes of the first 17 positive integers
• 23497 = cuban prime[9]
• 23821 = square pyramidal number[4]
• 23833 = Padovan prime
• 23969 = octahedral number[7]
• 23976 = pentagonal pyramidal number[2]

• 24211 = Zeisel number[11]
• 24336 = 156², palindromic in base 5: 1234321₅
• 24389 = 29³
• 24571 = cuban prime[9]
• 24601 – Jean Valjean's prisoner number in Les Misérables
• 24631 = Wedderburn–Etherington prime[12]
• 24649 = 157², palindromic in base 12: 12321₁₂
• 24737 = one of five remaining Seventeen or Bust numbers in the Sierpinski problem

• 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[11]
• 25117 = cuban prime[9]
• 25200 = highly composite number[1]
• 25205 = largest number whose factorial is less than 10¹⁰⁰⁰⁰⁰
• 25585 = square pyramidal number[4]

• 26214 = octahedral number[7]
• 26227 = cuban prime[9]
• 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 = 30³
• 27434 = square pyramidal number[4]
• 27559 = Zeisel number[11]
• 27648 = 1¹ × 2² × 3³ × 4⁴
• 27653 = Friedman prime
• 27720 = highly composite number;[1] smallest number divisible by the numbers 1 to 12 (there is no smaller number divisible by the numbers 1 to 11)
• 27846 = harmonic divisor number[13]
• 27889 = 167²

• 28158 = pentagonal pyramidal number[2]
• 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 = 169² = 13⁴ = 119² + 120², number that is simultaneously a square number and a centered square number, palindromic in base 12: 14641₁₂
• 28595 = octahedral number[7]
• 28657 = Fibonacci prime,[14] Markov prime[15]
• 28900 = 170², palindromic in base 13: 10201₁₃

• 29241 = 171², sum of the cubes of the first 18 positive integers
• 29341 = Carmichael number[16]
• 29370 = square pyramidal number[4]
• 29527 = Friedman prime
• 29531 = Friedman prime
• 29791 = 31³

There are 983 prime numbers between 20000 and 30000.