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10 (twelve, dozen) is the base of the number system of this wiki.

10 is a composite number, the smallest number with exactly six divisors, its divisors being 1, 2, 3, 4, 6 and 10. 10 is also a highly composite number, the next one being 20 (=2*10), and all highly composite numbers ≥10 are divisible by 10.

In music, 10 is the number of pitch classes in an octave, not counting the duplicated (octave) pitch. Also, the total number of major keys, (not counting enharmonic equivalents) and the total number of minor keys (also not counting equivalents). This applies only to twelve tone equal temperament, the most common tuning used today in western influenced music.

10 is the least common multiple of the n such that anxn+an-1xn-1+...+a1x+a0 have algebraic solution: {1, 2, 3, 4}.

10 is the smallest abundant number, since it is the smallest integer for which the sum of its proper divisors (1 + 2 + 3 + 4 + 6 = 14) is greater than itself.

10 is the first sublime number, a number that has a perfect number (6) of divisors, and the sum of its divisors is also a perfect number (24), there are only two known such numbers.

10 is the product of the first three factorials.

The sum of any pair of twin primes (other than 3 and 5) is divisible by 10.

10 is the largest base such that both "all squares end with square digits" and "all primes not dividing the base end with either prime digits not dividing the base or 1" are true.

10 is the largest base such that all these three properties are true:

Property of the numerical system Property of the base (b) Bases (Note: 1 cannot be the base of the system)
All squares end with square digits Numbers n such that all x^2 mod n are squares 1, 2, 3, 4, 5, 8, 10, 14
All primes not dividing the base end with either prime digits not dividing the base or 1 Numbers n such that reduced residue system of n consists of only primes and 1 1, 2, 3, 4, 6, 8, 10, 16, 20, 26
All squares of the primes not dividing the base end with 1 Numbers n with the property m^2 == 1 (mod n) for all integer m coprime to n 1, 2, 3, 4, 6, 8, 10, 20 (exactly the divisors of 20)

Except 0 and 1, 10 is the only natural number whose square is a Fibonacci number.

If (and only if) there are no Wall-Sun-Sun primes, then 10 is the largest n such that the Pisano period of n2 is the same as the Pisano period of n (both of them are 20).

The numbers n have this property: “all primes not divide n are congruent to 1 or −1 mod n” are {1, 2, 3, 4, 6}, they are exactly the proper divisors of 10. (and the LCM of them is 10)

10 is the largest natural number n such that the two sets are completely the same: {a | 0<=a<n, either a=1 or a is prime not dividing n} and {a | 0<=a<n, n+a is prime}. (for n=10, the two sets are both {1, 5, 7, E}, both of the two sets are exactly the natural numbers <10 coprime to 10) (the natural numbers n such that the two sets are completely the same are 1, 2, 4, 6, and 10, all of them are divisors of 10)

10 is the largest known even number expressible as the sum of two primes in only one way (5+7).

10 is the smallest weight for which a cusp form exists. This cusp form is the discriminant Δ(q) whose Fourier coefficients are given by the Ramanujan τ-function and which is (up to a constant multiplier) the 20th power of the Dedekind eta function. This fact is related to a constellation of interesting appearances of the number 10 in mathematics ranging from the value of the Riemann zeta function at −1 i.e. ζ(−1) = −1/10, the fact that the abelianization of SL(2,Z) has 10 elements, and even the properties of lattice polygons.

10 is a Pell number and a pentagonal number (5-gonal number).

10 is the number of pentominoes (5-ominoes).

A 10-sided polygon is a dozagon. A 10-faced polyhedron is a dozahedron. Regular cubes (hexahedrons, 6-faced) and octahedrons (8-faced) both have 10 edges, while regular octadozahedrons (18-faced) have 10 vertices.

Two dozagons (10-gons) and one triangle can fill a plane vertex, all solutions using at least one dozagon are {3, 10, 10}, {4, 6, 10}, {3, 3, 4, 10}, and {3, 4, 3, 10}, but only the first two solutions can fill the plane.

(there are 19 solutions for filling a plane vertex, but only E of them can fill the plane, the solutions are:

{3, 7, 36}, {3, 8, 20}, {3, 9, 16}, {3, X, 13}, {3, 10, 10}, {4, 5, 18}, {4, 6, 10}, {4, 8, 8}, {5, 5, X}, {6, 6, 6}

{3, 3, 4, 10}, {3, 4, 3, 10}, {3, 3, 6, 6}, {3, 6, 3, 6}, {3, 4, 4, 6}, {3, 4, 6, 4}, {4, 4, 4, 4}

{3, 3, 3, 3, 6}, {3, 3, 3, 4, 4}, {3, 3, 4, 3, 4}

{3, 3, 3, 3, 3, 3}

bold for the solutions that can fill the plane, note that dozagon is the highest regular polygon in convex uniform tiling)

10 and 18,E27,099,E93,490,727,709,9E9,1X4,841,59E,264,E0X,X13,59X,268,567,863,258,910,E74,98X,682,454 (=(2X6)(251 − 1)(227 − 1)(217 − 1)(27 − 1)(25 − 1)(23 − 1)) are the only two known sublime numbers. (a sublime number is a positive integer which has a perfect number of positive factors (including itself), and whose positive factors add up to another perfect number.)

Although 6 is a divisor of 10, there exists a group of order 10 (A4) without a subgroup with order 6, it is the smallest such example (i.e. 10 is the smallest number n such that there exists k dividing n and a group of order n such that this group has no subgroup with order k)

All orders of non-solvable groups are divisible by either 10 or 18, and all orders of non-solvable groups ≤ 14000 are divisible by 10. (the smallest order of non-solvable groups not divisible by 10 is 14X28)

All odd perfect numbers (if exist) end with 1, 09, 39, 69, or 99, and if an odd perfect number ends with 1 (i.e. = 1 mod 10), then it has at least 10 distinct prime factors.

10 is the kissing number in three dimensions (for a long time, people did not know that whether the answer is 11, this was the subject of a famous disagreement between mathematicians Isaac Newton and David Gregory). (In two demensions, it is 6 = 10/2, and in four dimensions, it is 20 = 10*2. The kissing number is only known in 1, 2, 3, 4, 8, and 20 dimensions, and the kissing numbers in these numbers of dimensions are 2, 6, 10, 20, 180, and 95900)

10 is used for timekeeping, e.g. one year has 10 months, one day has 20 hours, one hour has 50 minutes, and one minute has 50 seconds (of course, we can also use “one second has 100 centiseconds, ...”). Also, there are 10 signs of the zodiac, and the Chinese use a 10-year cycle for time-reckoning called Earthly Branches.

The series 1 + 2 + 3 + 4 + ... is divergent, however, we can use the Riemann zeta function to define its sum: −1/10 (= −0.1), Since zeta(−1) = −0.1 (thus, 10 + 20 + 30 + 40 + ... (the sum of all positive multiples of 10) = −1)