20,000

Selected numbers in the range 20001–29999

20001 to 20999

• 20002 = number of surface-points of a tetrahedron with edge-length 100
• 20100 = sum of the first 200 natural numbers (hence the 200th triangular number)
• 20160 = 23rd 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), also called a dozen great-gross in some duodecimal nomenclature.
• 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
• 21637 = number of partitions of 37
• 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 \leq 2^{18}.
• 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
• 24742 = number of signed trees with 10 nodes

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 = 224th triangular number, 24th highly composite number, smallest number with exactly 90 factors
• 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
• 25920 = smallest number with exactly 70 factors

26000 to 26999

• 26015 = number of partitions of 38
• 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
• 27405 = heptagonal number, hexadecagonal number, 48-gonal number, 80-gonal number, smallest integer that is polygonal in exactly 10 ways.
• 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 = 25th highly composite number; 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
• 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

References

1. {{cite OEIS. A005893. Number of points on surface of tetrahedron
2. {{Cite OEIS. A002182. Highly composite numbers
3. {{Cite OEIS. A002411. Pentagonal pyramidal numbers
4. {{Cite OEIS. A076980. Leyland numbers
5. {{Cite OEIS. A000330. Square pyramidal numbers
6. {{Cite OEIS. A000078. Tetranacci numbers
7. {{cite OEIS. A111441. Numbers k such that the sum of the squares of the first k primes is divisible by k
8. {{Cite OEIS. A000110. Bell or exponential numbers
9. {{cite OEIS. A000014. Number of series-reduced trees with n nodes
10. {{cite OEIS. A000041. a(n) is the number of partitions of n (the partition numbers)
11. {{Cite OEIS. A005900. Octahedral numbers
12. {{Cite OEIS. A006886. Kaprekar numbers
13. {{Cite OEIS. A002407. Cuban primes
14. {{Cite OEIS. A003261. Woodall numbers
15. {{cite OEIS. A007053. Number of primes [greater than or equal to] 2^n
16. {{cite OEIS. A011260. Number of primitive polynomials of degree n over GF(2)
17. {{Cite OEIS. A051015. Zeisel numbers
18. {{Cite OEIS. A001190. Wedderburn-Etherington numbers
19. {{cite OEIS. A000060. Number of signed trees with n nodes
20. "A002182 - OEIS".
21. {{cite OEIS. A000011. Number of n-bead necklaces (turning over is allowed) where complements are equivalent
22. {{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
23. {{cite OEIS. A000041. a(n) is the number of partitions of n (the partition numbers)
24. {{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
25. {{cite OEIS. A000566. Heptagonal numbers
26. {{cite OEIS. A051868. Hexadecagonal numbers
27. {{cite OEIS. A063778
28. "Sloane's A001599 : Harmonic or Ore numbers". OEIS Foundation.
29. "Sloane's A000045 : Fibonacci numbers". OEIS Foundation.
30. "Sloane's A002559 : Markoff (or Markov) numbers". OEIS Foundation.
31. "Sloane's A002997 : Carmichael numbers". OEIS Foundation.
32. {{cite OEIS. A000219. Number of planar partitions (or plane partitions) of n