Selected numbers

• 20000 – round number
  Round number
  A round number is mathematically defined as the product of a considerable number of comparatively small factors as compared to its neighbouring numbers, such as 24 = 2*2*2*3 .However, a round number is informally considered to be an integer that ends with one or more zeroes , such...
  
  ; also in the title of Jules Verne
  Jules Verne
  Jules Gabriel Verne was a French author who pioneered the science fiction genre. He is best known for his novels Twenty Thousand Leagues Under the Sea , A Journey to the Center of the Earth , and Around the World in Eighty Days...
  
  's novel 20,000 Leagues Under The Sea
• 20081 – Motorola 68K instruction for no operation (NOP)
• 20100 – sum of the first 200 natural numbers (hence a triangular number
  Triangular number
  A triangular number or triangle number numbers the objects that can form an equilateral triangle, as in the diagram on the right. The nth triangle number is the number of dots in a triangle with n dots on a side; it is the sum of the n natural numbers from 1 to n...
  
  )
• 20160 – the smallest order belonging to two non-isomorphic simple group
  Simple group
  In mathematics, a simple group is a nontrivial group whose only normal subgroups are the trivial group and the group itself. A group that is not simple can be broken into two smaller groups, a normal subgroup and the quotient group, and the process can be repeated...
  
  s: 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
  Pentagonal pyramidal number
  A pentagonal pyramidal number is a figurate number that represents the number of objects in a pyramid with a pentagonal base. The nth pentagonal pyramidal number is equal to the sum of the first n pentagonal numbers....
  
  
• 20412 – Leyland number
  Leyland number
  In number theory, a Leyland number is a number of the form xy + yx, where x and y are integers greater than 1. The first few Leyland numbers are...
  
  
• 20540 – square pyramidal number
  Square pyramidal number
  In mathematics, a pyramid number, or square pyramidal number, is a figurate number that represents the number of stacked spheres in a pyramid with a square base...
  
  
• 20569 – tetranacci number
• 20903 – first prime of form 120k+23 that is not a full reptend prime
• 21147 – Bell number
  Bell number
  In combinatorics, the nth Bell number, named after Eric Temple Bell, is the number of partitions of a set with n members, or equivalently, the number of equivalence relations on it...
  
  
• 21181 – one of six remaining Seventeen or Bust
  Seventeen or Bust
  Seventeen or Bust is a distributed computing project started in March 2002 to solve the last seventeen cases in the Sierpinski problem.-Goals:...
  
   numbers in the Sierpinski problem
• 21856 – octahedral number
  Octahedral number
  In number theory, an octahedral number is a figurate number that represents the number of spheres in an octahedron formed from close-packed spheres...
  
  
• 21978 - reverses when multiplied by 4
• 22050 – pentagonal pyramidal number
• 22140 – square pyramidal number
• 22222 – Kaprekar number
  Kaprekar number
  In mathematics, a Kaprekar number for a given base is a non-negative integer, the representation of whose square in that base can be split into two parts that add up to the original number again. For instance, 45 is a Kaprekar number, because 45² = 2025 and 20+25 = 45. The Kaprekar numbers are...
  
  
• 22447 – cuban prime
• 22527 – Woodall number
  Woodall number
  In number theory, a Woodall number is any natural number of the formfor some natural number n. The first few Woodall numbers are:Woodall numbers were first studied by Allan J. C. Cunningham and H. J. Woodall in 1917, inspired by James Cullen's earlier study of the similarly-defined Cullen numbers...
  
  
• 22699 – one of six remaining Seventeen or Bust
  Seventeen or Bust
  Seventeen or Bust is a distributed computing project started in March 2002 to solve the last seventeen cases in the Sierpinski problem.-Goals:...
  
   numbers in the Sierpinski problem
• 23401 – Leyland number
• 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
• 24211 – Zeisel number
• 24571 – cuban prime
• 24601 – Jean Valjean
  Jean Valjean
  Jean Valjean is the protagonist of Victor Hugo's 1862 novel Les Misérables...
  
  's prisoner number in Les Miserables
  Les Misérables
  Les Misérables , translated variously from the French as The Miserable Ones, The Wretched, The Poor Ones, The Wretched Poor, or The Victims), is an 1862 French novel by author Victor Hugo and is widely considered one of the greatest novels of the nineteenth century...
  
  
• 24631 – Wedderburn-Etherington number
  Wedderburn-Etherington number
  In graph theory, the Wedderburn–Etherington numbers, named for Ivor Malcolm Haddon Etherington and Joseph Wedderburn, count how many weak binary trees can be constructed: that is, the number of trees for which each graph vertex is adjacent to no more than three other such vertices, for a...
  
  
• 24737 – one of six remaining Seventeen or Bust
  Seventeen or Bust
  Seventeen or Bust is a distributed computing project started in March 2002 to solve the last seventeen cases in the Sierpinski problem.-Goals:...
  
   numbers in the Sierpinski problem
• 25011 – the first composite number that in base 10 remains composite after any insertion of a digit
• 25085 – Zeisel number
• 25117 – cuban prime
• 25205 – largest number whose factorial is less than 10¹⁰⁰⁰⁰⁰
• 25585 – square pyramidal number
• 26214 – octahedral number
• 26227 – cuban prime
• 26861 – smallest number below which there are more primes of the form 4k + 1 than of the form 4k + 3
• 27434 – square pyramidal number
• 27559 – Zeisel number
• 27648 – 1¹*2²*3³*4⁴
• 27720 – 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
• 28158 – pentagonal pyramidal number
  Pyramidal number
  A pyramidal number is a figurate number that represents a pyramid with a base and a given number of sides. The term is most often used to refer to square pyramidal numbers, which have four sides, but it can also refer to:...
  
  
• 28374 – smallest integer to start a run of six consecutive integers with the same number of divisors
• 28561 – , number that is simultaneously a square number
  Square number
  In mathematics, a square number, sometimes also called a perfect square, is an integer that is the square of an integer; in other words, it is the product of some integer with itself...
  
   and a centered square number
  Centered square number
  In elementary number theory, a centered square number is a centered figurate number that gives the number of dots in a square with a dot in the center and all other dots surrounding the center dot in successive square layers. That is, each centered square number equals the number of dots within a...
  
  
• 28595 – octahedral number
• 28657 – Fibonacci number, Markov number
• 29241 – sum of the cubes of the first 18 positive integers
• 29341 – Carmichael number
• 29370 – square pyramidal number