20,000

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 novel Twenty Thousand Leagues Under the Sea.
Selected numbers in the range 20001–29999
20001 to 20999
20002 = number of surface-points of a tetrahedron with edge-length 100
20067 = The smallest number with no entry in the Online Encyclopedia of Integer Sequences (OEIS)
20100 = sum of the first 200 natural numbers (hence a triangular number)
20160 = highly composite number; 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
20412 = Leyland number: 93 + 39
20540 = square pyramidal number
20569 = tetranacci number
20593 = unique prime in base 12
20597 = k such that the sum of the squares of the first k primes is divisible by k.
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
21181 = the least of five remaining Seventeen or Bust numbers in the Sierpiński problem
21209 = number of reduced trees with 23 nodes
21856 = octahedral number
21943 = Friedman prime
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
23000 = number of primes .
23401 = Leyland number: 65 + 56
23409 = 1532, sum of the cubes of the first 17 positive integers
23497 = cuban prime
23821 = square pyramidal number
23833 = Padovan prime
23969 = octahedral number
23976 = pentagonal pyramidal number
24000 to 24999
24000 = number of primitive polynomials of degree 20 over GF(2)
24211 = Zeisel number
24336 = 1562, palindromic in base 5: 12343215
24389 = 293
24571 = cuban prime
24631 = Wedderburn–Etherington prime
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 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
25117 = cuban prime
25200 = highly composite number
25205 = largest number whose factorial is less than 10100000
25482 = number of 21-bead necklaces (turning over is allowed) where complements are equivalent
25585 = square pyramidal number
25724 = Fine number
26000 to 26999
26214 = octahedral number
26227 = cuban prime
26272 = number of 20-bead binary necklaces with beads of 2 colors where the colors may be swapped but turning over is not allowed
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
27594 = number of primitive polynomials of degree 19 over GF(2)
27648 = 11 × 22 × 33 × 44
27653 = Friedman prime
27720 = highly composite number; 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
27889 = 1672
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
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
28657 = Fibonacci prime, Markov prime
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
29370 = square pyramidal number
29527 = Friedman prime
29531 = Friedman prime
29601 = number of planar partitions of 18
29791 = 313
Primes
There are 983 prime numbers between 20000 and 30000.
References
20000