72 (number)

72 (seventy-two) is the natural number following 71 and preceding 73. It is half a gross and also six dozen (i.e., 60 in duodecimal).

In mathematics

72 is a pronic number, as it is the product of 8 and 9. It is the smallest Achilles number, as it is a powerful number that is not itself a power.

Hipparchus (greek mathematician‐astronomer c.190 – c.120 BC) is purported to have discovered the phenomenon of the precession of the equinoxes by comparing the position of the vernal equinox against the fixed stars, noting that it shifts westward by about one degree every 72 years.

72 is an abundant number. With exactly twelve positive divisors, including 12 (one of only two sublime numbers), 72 is also the twelfth member in the sequence of refactorable numbers. As no smaller number has more than 12 divisors, 72 is a largely composite number. 72 has an Euler totient of 24. It is a highly totient number, as there are 17 solutions to the equation φ(x) = 72, more than any integer under 72. It is equal to the sum of its preceding smaller highly totient numbers 24 and 48, and contains the first six highly totient numbers 1, 2, 4, 8, 12 and 24 as a subset of its proper divisors. 144, or twice 72, is also highly totient, as is 576, the square of 24. While 17 different integers have a totient value of 72, the sum of Euler's totient function φ(x) over the first 15 integers is 72. It is also a perfect indexed Harshad number in decimal (twenty-eighth), as it is divisible by the sum of its digits (9).

72 plays a role in the rule of 72 in economics when approximating annual compounding of interest rates of a round 6% to 10%, due in part to its high number of divisors.

Inside Lie algebras:

• 72 is the number of vertices of the six-dimensional 1₂₂ polytope, which also contains as facets 720 edges, 702 polychoral 4-faces, of which 270 are four-dimensional 16-cells, and two sets of 27 demipenteract 5-faces. These 72 vertices are the root vectors of the simple Lie group , which as a honeycomb under 2₂₂ forms the lattice. 1₂₂ is part of a family of k₂₂ polytopes whose first member is the fourth-dimensional 3-3 duoprism, of symmetry order 72 and made of six triangular prisms. On the other hand, 3₂₁ ∈ k₂₁ is the only semiregular polytope in the seventh dimension, also featuring a total of 702 6-faces of which 576 are 6-simplexes and 126 are 6-orthoplexes that contain 60 edges and 12 vertices, or collectively 72 one-dimensional and two-dimensional elements; with 126 the number of root vectors in , which are contained in the vertices of 2₃₁ ∈ k₃₁, also with 576 or 24² 6-simplexes like 3₂₁. The triangular prism is the root polytope in the k₂₁ family of polytopes, which is the simplest semiregular polytope, with k₃₁ rooted in the analogous four-dimensional tetrahedral prism that has four triangular prisms alongside two tetrahedra as cells.
• The complex Hessian polyhedron in contains 72 regular complex triangular edges, as well as 27 polygonal Möbius–Kantor faces and 27 vertices. It is notable for being the vertex figure of the complex Witting polytope, which shares 240 vertices with the eight-dimensional semiregular 4₂₁ polytope whose vertices in turn represent the root vectors of the simple Lie group .

There are 72 compact and paracompact Coxeter groups of ranks four through ten: 14 of these are compact finite representations in only three-dimensional and four-dimensional spaces, with the remaining 58 paracompact or noncompact infinite representations in dimensions three through nine. These terminate with three paracompact groups in the ninth dimension, of which the most important is : it contains the final semiregular hyperbolic honeycomb 6₂₁ made of only regular facets and the 5₂₁ Euclidean honeycomb as its vertex figure, which is the geometric representation of the lattice. Furthermore, shares the same fundamental symmetries with the Coxeter-Dynkin over-extended form ⁺⁺ equivalent to the tenth-dimensional symmetries of Lie algebra .

72 lies between the 8th pair of twin primes (71, 73), where 71 is the largest supersingular prime that is a factor of the largest sporadic group (the friendly giant ), and 73 the largest indexed member of a definite quadratic integer matrix representative of all prime numbers that is also the number of distinct orders (without multiplicity) inside all 194 conjugacy classes of . Sporadic groups are a family of twenty-six finite simple groups, where , , and are associated exceptional groups that are part of sixteen finite Lie groups that are also simple, or non-trivial groups whose only normal subgroups are the trivial group and the groups themselves.

In literature

• In Plutarch's recounting of the Egyptian myth, 72 accomplices conspire with Set to assassinate Osiris.
• In the Hadith, sayings of Muhammad, 72 beautiful wives are promised to martyrs in paradise.
• Seventy-two scribes were said have transcribed the Septuagint (LXX) greek translation of what is today known as the Tanakh or Old Testament of the Bible at the request of Ptolemy II Philadelphus (285–247 BC).

In other fields

72 is also:

• In typography, a point is 1/72 inch.
• The rule of 72 in finance.
• 72 equal temperament is a tuning used in Byzantine music and by some modern composers.
• The number of micro seasons in the traditional Japanese calendar

Notes

References

External links

• Go Figure: What can 72 tell us about life, BBC News, 20 July 2011