20 (number)

Last updated
19 20 21
Cardinal twenty
Ordinal 20th
(twentieth)
Numeral system vigesimal
Factorization 22 × 5
Divisors 1, 2, 4, 5, 10, 20
Greek numeral Κ´
Roman numeral XX, xx
Binary 101002
Ternary 2023
Senary 326
Octal 248
Duodecimal 1812
Hexadecimal 1416
Armenian Ի
Hebrew כ / ך
Babylonian numeral
Egyptian hieroglyph 𓎏

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units is sometimes referred to as a score. [1] [2]

Contents

In Mathematics

Twenty is a composite number. It is also the smallest primitive abundant number. [3] The Happy Family of sporadic groups is made up of twenty finite simple groups that are all subquotients of the friendly giant, the largest of twenty-six sporadic groups.

Geometry

An icosagon is a polygon with 20 edges. Bring's curve is a Riemann surface, whose fundamental polygon is a regular hyperbolic icosagon. [4]

Platonic Solids

An icosahedron has twenty triangular faces. Icosahedron.svg
An icosahedron has twenty triangular faces.

The largest number of faces a Platonic solid can have is twenty faces, which make up a regular icosahedron. [5] A dodecahedron, on the other hand, has twenty vertices, likewise the most a regular polyhedron can have. [6] This is because the icosahedron and dodecahdron are duals of each other.

Other fields

Science

20 is the third magic number in physics.

Biology

In some countries, the number 20 is used as an index in measuring visual acuity. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision" in countries using the Imperial system. (The metric equivalent is 6/6.) When someone is able to see only after an event how things turned out, that person is often said to have had "20/20 hindsight" [7]

Psychology

In many disciplines of developmental psychology, adulthood starts at age 20. [8]

Culture

Age 20

The traditional age of majority in Japan, although the voting age has been reduced to 18. [9] Japanese people commemorate the twentieth birthday with personal ceremonies, and it comes with a number of legal rights like the right to marry. To represent this, the Japanese language has a special word for "20-years-old" that does not follow the rest of their numbering system. Accordingly, the word 二十歳 is read all at once as "はたち" (hatachi) rather than the expected pronunciation of the three characters as "にじゅうさい" (nijyuusai, which is literally "two," "ten," and the counter for "years old").

Number systems

20 is the basis for vigesimal number systems, used by several different civilizations in the past (and to this day), including the Maya. [10]

Indefinite number

A 'score' is a group of twenty (often used in combination with a cardinal number, e.g. fourscore to mean 80), [11] but also often used as an indefinite number [12] (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila"). [13]

Related Research Articles

Omega is the twenty-fourth and last letter in the Greek alphabet. In the Greek numeric system/isopsephy (gematria), it has a value of 800. The word literally means "great O", as opposed to omicron, which means "little O".

6 (six) is the natural number following 5 and preceding 7. It is a composite number and the smallest perfect number.

8 (eight) is the natural number following 7 and preceding 9.

A numerical digit or numeral is a single symbol used alone, or in combinations, to represent numbers in positional notation, such as the common base 10. The name "digit" originates from the Latin digiti meaning fingers.

22 (twenty-two) is the natural number following 21 and preceding 23.

24 (twenty-four) is the natural number following 23 and preceding 25. It is equal to two dozen and one sixth of a gross.

<span class="mw-page-title-main">Truncated dodecahedron</span> Archimedean solid with 32 faces

In geometry, the truncated dodecahedron is an Archimedean solid. It has 12 regular decagonal faces, 20 regular triangular faces, 60 vertices and 90 edges.

<span class="mw-page-title-main">Rhombic triacontahedron</span> Catalan solid with 30 faces

The rhombic triacontahedron, sometimes simply called the triacontahedron as it is the most common thirty-faced polyhedron, is a convex polyhedron with 30 rhombic faces. It has 60 edges and 32 vertices of two types. It is a Catalan solid, and the dual polyhedron of the icosidodecahedron. It is a zonohedron.

<span class="mw-page-title-main">Deltoidal hexecontahedron</span> Catalan polyhedron

In geometry, a deltoidal hexecontahedron is a Catalan solid which is the dual polyhedron of the rhombicosidodecahedron, an Archimedean solid. It is one of six Catalan solids to not have a Hamiltonian path among its vertices.

<span class="mw-page-title-main">Disdyakis triacontahedron</span> Catalan solid with 120 faces

In geometry, a disdyakis triacontahedron, hexakis icosahedron, decakis dodecahedron, kisrhombic triacontahedron or d120 is a Catalan solid with 120 faces and the dual to the Archimedean truncated icosidodecahedron. As such it is face-uniform but with irregular face polygons. It slightly resembles an inflated rhombic triacontahedron: if one replaces each face of the rhombic triacontahedron with a single vertex and four triangles in a regular fashion, one ends up with a disdyakis triacontahedron. That is, the disdyakis triacontahedron is the Kleetope of the rhombic triacontahedron. It is also the barycentric subdivision of the regular dodecahedron and icosahedron. It has the most faces among the Archimedean and Catalan solids, with the snub dodecahedron, with 92 faces, in second place.

Greek letters are used in mathematics, science, engineering, and other areas where mathematical notation is used as symbols for constants, special functions, and also conventionally for variables representing certain quantities. In these contexts, the capital letters and the small letters represent distinct and unrelated entities. Those Greek letters which have the same form as Latin letters are rarely used: capital A, B, E, Z, H, I, K, M, N, O, P, T, Y, X. Small ι, ο and υ are also rarely used, since they closely resemble the Latin letters i, o and u. Sometimes, font variants of Greek letters are used as distinct symbols in mathematics, in particular for ε/ϵ and π/ϖ. The archaic letter digamma (Ϝ/ϝ/ϛ) is sometimes used.

<span class="mw-page-title-main">Final stellation of the icosahedron</span> Outermost stellation of the icosahedron

In geometry, the complete or final stellation of the icosahedron is the outermost stellation of the icosahedron, and is "complete" and "final" because it includes all of the cells in the icosahedron's stellation diagram. That is, every three intersecting face planes of the icosahedral core intersect either on a vertex of this polyhedron or inside of it. It was studied by Max Brückner after the discovery of Kepler–Poinsot polyhedron. It can be viewed as an irregular, simple, and star polyhedron.

<span class="mw-page-title-main">Small stellated dodecahedron</span> A Kepler-Poinsot polyhedron

In geometry, the small stellated dodecahedron is a Kepler–Poinsot polyhedron, named by Arthur Cayley, and with Schläfli symbol {52,5}. It is one of four nonconvex regular polyhedra. It is composed of 12 pentagrammic faces, with five pentagrams meeting at each vertex.

<span class="mw-page-title-main">Rhombic icosahedron</span>

The rhombic icosahedron is a polyhedron shaped like an oblate sphere. Its 20 faces are congruent golden rhombi; 3, 4, or 5 faces meet at each vertex. It has 5 faces (green on top figure) meeting at each of its 2 poles; these 2 vertices lie on its axis of 5-fold symmetry, which is perpendicular to 5 axes of 2-fold symmetry through the midpoints of opposite equatorial edges (example on top figure: most left-hand and most right-hand mid-edges). Its other 10 faces follow its equator, 5 above and 5 below it; each of these 10 rhombi has 2 of its 4 sides lying on this zig-zag skew decagon equator. The rhombic icosahedron has 22 vertices. It has D5d, [2+,10], (2*5) symmetry group, of order 20; thus it has a center of symmetry (since 5 is odd).

<span class="mw-page-title-main">Operation (mathematics)</span> Addition, multiplication, division, ...

In mathematics, an operation is a function from a set to itself. For example, an operation on real numbers will take in real numbers and return a real number. An operation can take zero or more input values to a well-defined output value. The number of operands is the arity of the operation.

<span class="mw-page-title-main">Isohedral figure</span> ≥2-dimensional tessellation or ≥3-dimensional polytope with identical faces

In geometry, a tessellation of dimension 2 or higher, or a polytope of dimension 3 or higher, is isohedral or face-transitive if all its faces are the same. More specifically, all faces must be not merely congruent but must be transitive, i.e. must lie within the same symmetry orbit. In other words, for any two faces A and B, there must be a symmetry of the entire figure by translations, rotations, and/or reflections that maps A onto B. For this reason, convex isohedral polyhedra are the shapes that will make fair dice.

<span class="mw-page-title-main">Regular dodecahedron</span> Convex polyhedron with 12 regular pentagonal faces

A regular dodecahedron or pentagonal dodecahedron is a dodecahedron composed of regular pentagonal faces, three meeting at each vertex. It is an example of Platonic solids, described as cosmic stellation by Plato in his dialogues, and it was used as part of Solar System proposed by Johannes Kepler. However, the regular dodecahedron, including the other Platonic solids, has already been described by other philosophers since antiquity.

<span class="mw-page-title-main">Petrie polygon</span> Skew polygon derived from a polytope

In geometry, a Petrie polygon for a regular polytope of n dimensions is a skew polygon in which every n – 1 consecutive sides belongs to one of the facets. The Petrie polygon of a regular polygon is the regular polygon itself; that of a regular polyhedron is a skew polygon such that every two consecutive sides belongs to one of the faces. Petrie polygons are named for mathematician John Flinders Petrie.

The tee, also called down tack or verum, is a symbol used to represent:

References

    1. John H. Conway and Richard K. Guy, The Book of Numbers. New York: Copernicus (1996): 11. ""Score" is related to "share" and comes from the Old Norse "skor" meaning a "notch" or "tally" on a stick used for counting. ... Often people counted in 20s; every 20th notch was larger, so "score" also came to mean 20."
    2. "score | Origin and meaning of score by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-08-16.
    3. "Sloane's A071395 : Primitive abundant numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
    4. Weber, Matthias (2005). "Kepler's small stellated dodecahedron as a Riemann surface" (PDF). Pacific Journal of Mathematics . 220 (1): 172. doi: 10.2140/pjm.2005.220.167 . MR   2195068. S2CID   54518859. Zbl   1100.30036.
    5. Weisstein, Eric W. "Icosahedron". mathworld.wolfram.com. Retrieved 2020-08-16.
    6. Weisstein, Eric W. "Dodecahedron". mathworld.wolfram.com. Retrieved 2020-08-16.
    7. "Definition of 20/20". www.merriam-webster.com. Retrieved 2020-08-16.
    8. "Adulthood | Introduction to Psychology". lumenlearning.com.
    9. "Japan's Age of Majority Changed to 18 - Living the Japon.com". www.japan-experience.com. Retrieved 19 March 2018.
    10. Weisstein, Eric W. "Vigesimal". mathworld.wolfram.com. Retrieved 2020-08-16.
    11. "Definition of SCORE". www.merriam-webster.com. Retrieved 2020-08-16.
    12. "Biblical Criticism", The Classical Journal36:71:83ff (March 1827) full text
    13. "CBS News", Scores of Typhoon Survivors Flown to Manila (November 2013)