24 (number)

24 (twenty-four) is the natural number following 23 and preceding 25.

The SI prefix for 10²⁴ is yotta (Y), and for 10⁻²⁴ (i.e., the reciprocal of 10²⁴) yocto (y). These numbers are the largest and smallest number to receive an SI prefix to date. In a 24-hour clock, the twenty-fourth hour is in conventional language called twelve or twelve o'clock.

In mathematics

• 24 is the factorial of 4 (24 = 4!) and a composite number, being the first number of the form 2³q, where q is an odd prime.
• Since 24 = 4!, it follows that 24 is the number of ways to order 4 distinct items: (1,2,3,4), (1,2,4,3), (1,3,2,4), (1,3,4,2), (1,4,2,3), (1,4,3,2), (2,1,3,4), (2,1,4,3), (2,3,1,4), (2,3,4,1), (2,4,1,3), (2,4,3,1), (3,1,2,4), (3,1,4,2), (3,2,1,4), (3,2,4,1), (3,4,1,2), (3,4,2,1), (4,1,2,3), (4,1,3,2), (4,2,1,3), (4,2,3,1), (4,3,1,2), (4,3,2,1).
• It is the smallest number with exactly eight divisors: 1, 2, 3, 4, 6, 8, 12, and 24.
• It is a highly composite number, having more divisors than any smaller number.^[1]
• 24 is a semiperfect number, since adding up all the proper divisors of 24 except 4 and 8 gives 24.^[2]
• Subtracting 1 from any of its divisors (except 1 and 2, but including itself) yields a prime number; 24 is the largest number with this property.
• There are 10 solutions to the equation φ(x) = 24, namely 35, 39, 45, 52, 56, 70, 72, 78, 84 and 90. This is more than any integer below 24, making 24 a highly totient number.^[3]
• 24 is a nonagonal number.^[4]
• 24 is the sum of the prime twins 11 and 13.
• 24 is a Harshad number.^[5]
• 24 is a semi-meandric number.
• The product of any four consecutive numbers is divisible by 24. This is because among any four consecutive numbers there must be two even numbers, one of which is a multiple of four, and there must be a multiple of three.
• The tesseract has 24 two-dimensional faces (which are all squares).
• 24 is the only nontrivial solution to the cannonball problem, that is: 1² + 2² + 3² + … + 24² is a perfect square (70²). (The trivial case is just 1² = 1².)
• In 24 dimensions there are 24 even positive definite unimodular lattices, called the Niemeier lattices. One of these is the exceptional Leech lattice which has many surprising properties; due to its existence, the answers to many problems such as the kissing number problem and densest lattice sphere-packing problem are known in 24 dimensions but not in many lower dimensions. The Leech lattice is closely related to the equally nice length-24 binary Golay code and the Steiner system S(5,8,24) and the Mathieu group M₂₄. (One construction of the Leech lattice is possible because 1²+2²+3²+...+24² =70².)
• The modular discriminant Δ(τ) is proportional to the 24th power of the Dedekind eta function η(τ):       Δ(τ) = (2π)¹²η(τ)²⁴.
• The Barnes-Wall lattice contains 24 lattices.
• 24 is the only number whose divisors — namely 1, 2, 3, 4, 6, 8, 12, 24 — are exactly those numbers n for which every invertible element of the commutative ring Z/nZ is a square root of 1. It follows that the multiplicative group (Z/24Z)^× = {±1, ±5, ±7, ±11} is isomorphic to the additive group (Z/2Z)³. This fact plays a role in monstrous moonshine.

  It follows that any number n relatively prime to 24, and in particular any prime n greater than 3, has the property that n² – 1 is divisible by 24.

• The 24-cell, with 24 octahedral cells and 24 vertices, is a self-dual convex regular 4-polytope; it has no analogue in any other dimension. Its 24 vertices can be expressed as the set {±1, ±i, ±j, ±k, ±1 ± i ± j ± k/2} of unit quaternions, using all choices of signs. This set forms a group under quaternion multiplication, isomorphic to the binary tetrahedral group. The quotient of the unit quaternions S³ by this subgroup is identical as a metric space to the configuration space of a regular tetrahedron centered at the origin in 3-space. The 24-cell tiles 4-dimensional space.
• 24 is the kissing number in 4-dimensional space: the maximum number of unit spheres that can all touch another unit sphere without overlapping. (The centers of 24 such spheres form the vertices of a 24-cell.)
• 24 is the largest integer that is divisible by all natural numbers no larger than its square root.
• 24 is the Euler characteristic of a K3 surface
• 24 is the smallest 5-hemiperfect number
• 24 is the order of the cyclic group equal to the stable 3-stem in homotopy groups of spheres: π_n+3(Sⁿ) = Z/24Z for all n ≥ 5.

In science

• The atomic number of chromium
• The number of hours in a day
• 24! is an approximation (exceeding by just over 3%) of the Avogadro constant.

In religion

• The number of books in the Tanakh.
• In Christian apocalyptic literature it represents the complete Church, being the sum of the 12 tribes of Israel and the 12 Apostles of the Lamb of God .
• Number of Tirthankaras.
• Number of spokes in the Ashok Chakra.

In music

• There are a total of 24 major and minor keys in Western tonal music, not counting enharmonic equivalents. Therefore, for collections of pieces written in each key, the number of pieces in such a collection; e.g., Chopin's 24 Preludes.

In sports

• Four-and-Twenty was an American racehorse.

In other fields

Astronomical clock in Prague

24 is also:

• The number of bits a computer needs to represent TrueColor images (for a maximum of 16,777,216 colours). (But greater numbers of bits provide more accurate colors. "TrueColor" is one of many possible representations of colors.)
• The number of carats representing 100% pure gold.
• The number of cycles in the Chinese solar year.
• The number of frames per second at which motion picture film is usually projected.
• The number of letters in both the modern and classical Greek alphabet. For the latter reason, also the number of chapters or "books" into which Homer's Odyssey and Iliad came to be divided.
• The number of points on a backgammon board.
• A children's mathematical game involving the use of any of the four standard operations on four numbers on a card to get 24 (see Math 24)
• The maximum number of Knight Companions in the Order of the Garter
• The number of the French department Dordogne.
• Four and twenty is the number of blackbirds baked in a pie in the traditional English nursery rhyme Sing a Song of Sixpence.

References

External links

Wikimedia Commons has media related to 24 (number).

• My Favorite Numbers: 24, John C. Baez