20001 to 20999

• 20002 = number of surface-points of a tetrahedron with edge-length 100^[1]
• 20067 = The smallest number with no entry in the Online Encyclopedia of Integer Sequences (OEIS)Bischoff, Manon (March 3, 2023). "The Most Boring Number in the World Is ..." Scientific American. Springer Nature.
• 20100 = sum of the first 200 natural numbers (hence a triangular number)
• 20160 = highly composite number;^[2] 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^[3]
• 20412 = Leyland number:^[4] 9³ + 3⁹
• 20540 = square pyramidal number^[5]
• 20569 = tetranacci number^[6]
• 20593 = unique prime in base 12
• 20597 = k such that the sum of the squares of the first k primes is divisible by k.^[7]
• 20736 = 144² = 12⁴, 10000₁₂, palindromic in base 15 (6226₁₅)
• 20793 = little Schroeder number
• 20871 = The number of weeks in exactly 400 years in the Gregorian calendar
• 20903 = first prime of form 120k + 23 that is not a full reptend prime

21000 to 21999

• 21025 = 145², palindromic in base 12 (10201₁₂)
• 21147 = Bell number^[8]
• 21181 = the least of five remaining Seventeen or Bust numbers in the Sierpiński problem
• 21209 = number of reduced trees with 23 nodes^[9]
• 21637 = number of partitions of 37^[10]
• 21856 = octahedral number^[11]
• 21943 = Friedman prime
• 21952 = 28³
• 21978 = reverses when multiplied by 4: 4 × 21978 = 87912

22000 to 22999

• 22050 = pentagonal pyramidal number^[3]
• 22140 = square pyramidal number^[5]
• 22222 = repdigit, Kaprekar number:^[12] 22222² = 493817284, 4938 + 17284 = 22222
• 22447 = cuban prime^[13]
• 22527 = Woodall number: 11 × 2¹¹ − 1^[14]
• 22621 = repunit prime in base 12
• 22699 = one of five remaining Seventeen or Bust numbers in the Sierpiński problem

23000 to 23999

• 23000 = number of primes ${\displaystyle \leq 2^{18}}$.^[15]
• 23401 = Leyland number:^[4] 6⁵ + 5⁶
• 23409 = 153², sum of the cubes of the first 17 positive integers
• 23497 = cuban prime^[13]
• 23821 = square pyramidal number^[5]
• 23833 = Padovan prime
• 23969 = octahedral number^[11]
• 23976 = pentagonal pyramidal number^[3]

24000 to 24999

• 24000 = number of primitive polynomials of degree 20 over GF(2)^[16]
• 24211 = Zeisel number^[17]
• 24336 = 156², palindromic in base 5: 1234321₅
• 24389 = 29³
• 24571 = cuban prime^[13]
• 24631 = Wedderburn–Etherington prime^[18]
• 24649 = 157², palindromic in base 12: 12321₁₂
• 24737 = one of five remaining Seventeen or Bust numbers in the Sierpinski problem
• 24742 = number of signed trees with 10 nodes^[19]

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^[17]
• 25117 = cuban prime^[13]
• 25200 = highly composite number, smallest number with exactly 90 factors^[2]
• 25205 = largest number whose factorial is less than 10¹⁰⁰⁰⁰⁰
• 25482 = number of 21-bead necklaces (turning over is allowed) where complements are equivalent^[20]
• 25585 = square pyramidal number^[5]
• 25724 = Fine number^[21]
• 25920 = smallest number with exactly 70 factors

26000 to 26999

• 26015 = number of partitions of 38^[22]
• 26214 = octahedral number^[11]
• 26227 = cuban prime^[13]
• 26272 = number of 20-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed^[23]
• 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
• 27405 = heptagonal number,^[24] hexadecagonal number,^[25] 48-gonal number, 80-gonal number, smallest integer that is polygonal in exactly 10 ways.^[26]
• 27434 = square pyramidal number^[5]
• 27559 = Zeisel number^[17]
• 27594 = number of primitive polynomials of degree 19 over GF(2)^[16]
• 27648 = 1¹ × 2² × 3³ × 4⁴
• 27653 = Friedman prime
• 27720 = highly composite number;^[2] smallest number divisible by the numbers 1 to 12 (there is no smaller number divisible by the numbers 1 to 11 since any number divisible by 4 and 3 must be divisible by 12, since 4×3=12)
• 27846 = harmonic divisor number^[27]
• 27889 = 167²

28000 to 28999

• 28158 = pentagonal pyramidal number^[3]
• 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^[11]
• 28657 = Fibonacci prime,^[28] Markov prime^[29]
• 28900 = 170², palindromic in base 13: 10201₁₃

29000 to 29999

• 29241 = 171², sum of the cubes of the first 18 positive integers
• 29341 = Carmichael number^[30]
• 29370 = square pyramidal number^[5]
• 29527 = Friedman prime
• 29531 = Friedman prime
• 29601 = number of planar partitions of 18^[31]
• 29791 = 31³

Primes

There are 983 prime numbers between 20000 and 30000.