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**10** (**ten**) is the even natural number following 9 and preceding 11. Ten is the base of the decimal numeral system, the most common system of denoting numbers in both spoken and written language.

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Cardinal | ten | |||

Ordinal | 10th (tenth) | |||

Numeral system | decimal | |||

Factorization | 2 × 5 | |||

Divisors | 1, 2, 5, 10 | |||

Greek numeral | Ι´ | |||

Roman numeral | X | |||

Roman numeral (unicode) | X, x | |||

Greek prefix | deca-/deka- | |||

Latin prefix | deci- | |||

Binary | 1010_{2} | |||

Ternary | 101_{3} | |||

Senary | 14_{6} | |||

Octal | 12_{8} | |||

Duodecimal | A_{12} | |||

Hexadecimal | A_{16} | |||

Chinese numeral | 十，拾 | |||

Hebrew | י (Yod) | |||

Khmer | ១០ | |||

Armenian | Ժ | |||

Tamil | ௰ | |||

Thai | ๑๐ | |||

Devanāgarī | १० | |||

Bengali | ১০ | |||

Arabic & Kurdish & Iranian | ١٠ | |||

Malayalam | ൰ | |||

Egyptian hieroglyph | 𓎆 | |||

Babylonian numeral | 𒌋 |

- A collection of ten items (most often ten years) is called a decade.
- The ordinal adjective is
*decimal*; the distributive adjective is*denary*. - Increasing a quantity by one order of magnitude is most widely understood to mean multiplying the quantity by ten.
- To reduce something by one tenth is to
*decimate*. (In ancient Rome, the killing of one in ten soldiers in a cohort was the punishment for cowardice or mutiny; or, one-tenth of the able-bodied men in a village as a form of retribution, thus causing a labor shortage and threat of starvation in agrarian societies.)

**Ten** is the fifth composite number, and the smallest noncototient, which is a number that cannot be expressed as the difference between any integer and the total number of coprimes below it.^{[1]} Ten is the eighth Perrin number, preceded by 5, 5, and 7.^{[2]}

As important sums,

- , the sum of the squares of the first two odd numbers
^{[3]} - , the sum of the first four positive integers, equivalently the fourth triangle number
^{[4]} - , the smallest number that can be written as the sum of two prime numbers in two different ways
^{[5]}^{[6]} - , the sum of the first three prime numbers, and the smallest semiprime that is the sum of all the distinct prime numbers from its lower factor through its higher factor
^{[7]}

The factorial of ten is equal to the product of the factorials of the first four odd numbers as well: ,^{[8]} and 10 is the only number whose sum and difference of its prime divisors yield prime numbers and .

- 10 is also the first number whose fourth power (10,000) can be written as a sum of two squares in two different ways, and

Ten has an aliquot sum of 8, and is the first discrete semiprime to be in deficit, as with all subsequent discrete semiprimes.^{[9]} It is the second composite in the aliquot sequence for ten (10, 8, 7, 1, 0) that is rooted in the prime **7**-aliquot tree.^{[10]}

It is a largely composite number,^{[11]} as it has 4 divisors and no smaller number has more than 4 divisors.

According to conjecture, ten is the average sum of the proper divisors of the natural numbers if the size of the numbers approaches infinity,^{[12]} and it is the smallest number whose status as a possible friendly number is unknown.^{[13]}

- The smallest integer with exactly ten divisors is 48, while the least integer with exactly eleven divisors is 1024, which sets a new record.
^{[14]}^{[a]}

Figurate numbers that represent regular ten-sided polygons are called decagonal and centered decagonal numbers.^{[15]} On the other hand, 10 is the first non-trivial centered triangular number^{[16]} and tetrahedral number.^{[17]} 10 is also the first member in the coordination sequence for body-centered tetragonal lattices.^{[18]}^{[19]}^{[b]}

- While
**55**is the tenth triangular number, it is also the tenth Fibonacci number, and the largest such number to also be a triangular number.^{[20]}55 is also the fourth doubly triangular number.^{[21]}

10 is the fourth telephone number, and the number of Young tableaux with four cells.^{[22]} It is also the number of -queens problem solutions for .^{[23]}

There are precisely ten small Pisot numbers that do not exceed the golden ratio.^{[24]}

As a constructible polygon with a compass and straight-edge, the regular **decagon** has an internal angle of degrees and a central angle of degrees. All regular -sided polygons with up to ten sides are able to tile a plane-vertex alongside other regular polygons alone; the first regular polygon unable to do so is the eleven-sided hendecagon.^{[25]}^{[c]} While the regular decagon cannot tile alongside other regular figures, ten of the eleven regular and semiregular tilings of the plane are Wythoffian (the elongated triangular tiling is the only exception);^{[26]} however, the plane can be *covered* using overlapping decagons, and is equivalent to the Penrose P2 tiling when it is decomposed into kites and rhombi that are proportioned in *golden ratio*.^{[27]} The regular decagon is also the Petrie polygon of the regular dodecahedron and icosahedron, and it is the largest face that an Archimedean solid can contain, as with the truncated dodecahedron and icosidodecahedron.^{[d]}

There are ten regular star polychora in the fourth dimension, all of which have orthographic projections in the Coxeter plane that contain various decagrammic symmetries, which include compound forms of the regular decagram.^{[28]}

is a multiply transitive permutation group on ten points. It is an almost simple group, of order,

It functions as a point stabilizer of degree 11 inside the smallest sporadic simple group , a group with an irreducible faithful complex representation in ten dimensions, and an order equal to that is one more than the *one-thousandth* prime number, 7919.

is an infinite-dimensional Kac–Moody algebra which has the even Lorentzian unimodular lattice II_{9,1} of dimension 10 as its root lattice. It is the first Lie algebra with a negative Cartan matrix determinant, of −1.

There are precisely ten affine Coxeter groups that admit a formal description of reflections across dimensions in Euclidean space. These contain *infinite* facets whose quotient group of their normal abelian subgroups is finite. They include the one-dimensional Coxeter group [**∞**], which represents the apeirogonal tiling, as well as the five affine Coxeter groups , , , , and that are associated with the five exceptional Lie algebras. They also include the four general affine Coxeter groups , , , and that are associated with simplex, cubic and demihypercubic honeycombs, or tessellations. Regarding Coxeter groups in hyperbolic space, there are infinitely many such groups; however, ten is the highest rank for paracompact hyperbolic solutions, with a representation in nine dimensions. There also exist hyperbolic Lorentzian *cocompact* groups where removing any permutation of two nodes in its Coxeter–Dynkin diagram leaves a finite or Euclidean graph. The tenth dimension is the highest dimensional representation for such solutions, which share a root symmetry in eleven dimensions. These are of particular interest in M-theory of string theory.

The SI prefix for 10 is "deca-".

The meaning "10" is part of the following terms:

Also, the number 10 plays a role in the following:

- The atomic number of neon.
- The number of hydrogen atoms in butane, a hydrocarbon.
- The number of spacetime dimensions in some superstring theories.

The metric system is based on the number 10, so converting units is done by adding or removing zeros (e.g. 1 centimetre = 10 millimetres, 1 decimetre = 10 centimetres, 1 meter = 100 centimetres, 1 dekametre = 10 meters, 1 kilometre = 1,000 meters).

- The interval of a major tenth is an octave plus a major third.
- The interval of a minor tenth is an octave plus a minor third.

The Ten Commandments in the Hebrew Bible are ethical commandments decreed by God (to Moses) for the people of Israel to follow.

- In Pythagoreanism, the number 10 played an important role and was symbolized by the tetractys.

- There are Ten Sephirot in the Kabbalistic Tree of Life.

- In Chinese astrology, the 10 Heavenly Stems, refer to a cyclic number system that is used also for time reckoning.

- Mathematics portal
- List of highways numbered 10

**^**The initial largest span of numbers for a new maximum record of divisors to appear lies between numbers with 1 and 5 divisors, respectively.

This is also the next greatest such span, set by the numbers with 7 and 11 divisors, and followed by numbers with 13 and 17 divisors; these are maximal records set by successive prime counts.

Powers of 10 contain divisors, where is the number of digits: 10 has 2^{2}= 4 divisors, 10^{2}has 3^{2}= 9 divisors, 10^{3}has 4^{2}= 16 divisors, and so forth.**^**Also found by

- "... reading the segment (1, 10) together with the line from 10, in the direction 10, 34, ..., in the square spiral whose vertices are the generalized hexagonal numbers (A000217)."
^{[18]}

^{[3]}- "... reading the segment (1, 10) together with the line from 10, in the direction 10, 34, ..., in the square spiral whose vertices are the generalized hexagonal numbers (A000217)."
**^**Specifically, a decagon can fill a plane-vertex alongside two regular pentagons, and alongside a fifteen-sided pentadecagon and triangle.**^**The decagon is the*hemi-face*of the icosidodecahedron, such that a plane dissection yields two mirrored pentagonal rotundae. A regular ten-pointed {10/3} decagram is the hemi-face of the great icosidodecahedron, as well as the Petrie polygon of two regular Kepler–Poinsot polyhedra.

In total,**ten**non-prismatic uniform polyhedra contain regular decagons as faces (U_{26}, U_{28}, U_{33}, U_{37}, U_{39}, ...), and**ten**contain regular decagrams as faces (U_{42}, U_{45}, U_{58}, U_{59}, U_{63}, ...). Also, the decagonal prism is the largest prism that is a facet inside four-dimensional uniform polychora.

**^**"Sloane's A005278 : Noncototients".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-06-01.**^**Sloane, N. J. A. (ed.). "Sequence A001608 (Perrin sequence (or Ondrej Such sequence))".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.- ^
^{a}^{b}Sloane, N. J. A. (ed.). "Sequence A108100 ((2*n-1)^2+(2*n+1)^2.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-11-07. **^**Sloane, N. J. A. (ed.). "Sequence A000217 (Triangular numbers: a(n) is the binomial(n+1,2) equal to n*(n+1)/2 or 0 + 1 + 2 + ... + n.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-12-02.**^**Sloane, N. J. A. (ed.). "Sequence A001172 (Smallest even number that is an unordered sum of two odd primes in exactly n ways.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-11-07.**^**Sloane, N. J. A. (ed.). "Sequence A067188 (Numbers that can be expressed as the (unordered) sum of two primes in exactly two ways.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-11-07.**^**Sloane, N. J. A. (ed.). "Sequence A055233 (Composite numbers equal to the sum of the primes from their smallest prime factor to their largest prime factor.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.**^**"10".*PrimeCurios!*. PrimePages. Retrieved 2023-01-14.**^**Sloane, N. J. A. (ed.). "Sequence A001065 (Sum of proper divisors (or aliquot parts) of n: sum of divisors of n that are less than n.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.**^**Sloane, N. J. A. (1975). "Aliquot sequences".*Mathematics of Computation*.**29**(129). OEIS Foundation: 101–107. Retrieved 2022-12-08.**^**Sloane, N. J. A. (ed.). "Sequence A067128 (Ramanujan's largely composite numbers)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A297575 (Numbers whose sum of divisors is divisible by 10.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.**^**Sloane, N. J. A. (ed.). "Sequence A074902 (Known friendly numbers)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.**^**Sloane, N. J. A. (ed.). "Sequence A005179 (Smallest number with exactly n divisors.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-11-07.**^**"Sloane's A001107 : 10-gonal (or decagonal) numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-06-01.**^**"Sloane's A005448 : Centered triangular numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-06-01.**^**"Sloane's A000292 : Tetrahedral numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-06-01.- ^
^{a}^{b}Sloane, N. J. A. (ed.). "Sequence A008527 (Coordination sequence for body-centered tetragonal lattice.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-11-07. **^**O'Keeffe, Michael (1995). "Coordination sequences for lattices" (PDF).*Zeitschrift für Kristallographie*.**210**(12). Berlin: De Grutyer: 905–908. Bibcode:1995ZK....210..905O. doi:10.1524/zkri.1995.210.12.905. S2CID 96758246.**^**Sloane, N. J. A. (ed.). "Sequence A000217 (Triangular numbers)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.**^**Sloane, N. J. A. (ed.). "Sequence A002817 (Doubly triangular numbers: a(n) as n*(n+1)*(n^2+n+2)/8.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-12-18.**^**Sloane, N. J. A. (ed.). "Sequence A000085 (Number of self-inverse permutations on n letters, also known as involutions; number of standard Young tableaux with four cells;)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2023-02-17.**^**Sloane, N. J. A. (ed.). "Sequence A000170 (Number of ways of placing n nonattacking queens on an n X n board.)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2022-12-08.**^**M.J. Bertin; A. Decomps-Guilloux; M. Grandet-Hugot; M. Pathiaux-Delefosse; J.P. Schreiber (1992).*Pisot and Salem Numbers*. Birkhäuser. ISBN 3-7643-2648-4.**^**Grünbaum, Branko; Shepard, Geoffrey (November 1977). "Tilings by Regular Polygons" (PDF).*Mathematics Magazine*.**50**(5). Taylor & Francis, Ltd.: 230, 231. doi:10.2307/2689529. JSTOR 2689529. S2CID 123776612. Zbl 0385.51006.**^**Grünbaum, Branko; Shephard, G. C. (1987). "Section 2.1: Regular and uniform tilings".*Tilings and Patterns*. New York: W. H. Freeman and Company. p. 64. doi:10.2307/2323457. ISBN 0-7167-1193-1. JSTOR 2323457. OCLC 13092426. S2CID 119730123.**^**Gummelt, Petra (1996). "Penrose tilings as coverings of congruent decagons".*Geometriae Dedicata*.**62**(1). Berlin: Springer: 1–17. doi:10.1007/BF00239998. MR 1400977. S2CID 120127686. Zbl 0893.52011.**^**Coxeter, H. S. M (1948). "Chapter 14: Star-polytopes".*Regular Polytopes*. London: Methuen & Co. LTD. p. 263.

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