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Number 21 (twenty-one) is an odd two-digits composite number and natural number following 20 and preceding 22.

Properties of the number 21

Number of digits2
Sum of digits3
Product of digits2
Number parityOdd
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Prime Factorization of 21

Prime factorization3 x 7
Prime factors3, 7
Number of distinct prime factors ω(n)2
Total number of prime factors Ω(n)2
Sum of prime factors10
Product of prime factors21
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Divisors of 21

List of proper divisors 1, 3, 7
List of all dividers1, 3, 7, 21
Number of divisors d(n)4
Sum of all divisors σ(n)32
Aliquot sum 11
21 is a deficient number , since it is larger than the sum of its proper divisors (11). Its deficiency is 10.
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Prime numbers

Is 21 a prime number?No
Is 21 a semiprime number?Yes
Is 21 a Chen prime number?No
Is 21 a Mersenne prime number?No
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Is 21 a Catalan number?No
Is 21 a Fibonacci number?Yes
Is 21 a Idoneal number?Yes: I15
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Number theory

Number theory is a branch of pure mathematics devoted primarily to the study of the integers and integer-valued functions.
21 is not a Happy number
21 is a Humble number
21 is not a Münchhausen number
21 is not a Perfect number
21 is not a Perfect square
21 is not a Palindrome
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Numeric Bases of 21

In mathematics, a base or radix of a numeral system is the number of unique digits, including zero, used to represent numbers in that system.
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Mathematical operations

Mathematical operations are actions that are performed on numbers or mathematical expressions to find a numerical result.
Square of 21 (n2)441
Cube of 21 (n3)9261
Square root of 214.5825756949558
Natural Logarithm (ln) of 213.0445224377234
Decimal Logarithm (log) of 211.3222192947339
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Trigonometry is the study of the relationship between the angles and sides of a triangle.
Sine of 210.83665563853606
Cosecant of 211.1952348779359
Cosine of 21-0.54772926022427
Secant of 21-1.825719516227
Tangent of 21-1.5274985276366
Cotangent of 21-0.65466511548606
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Number parity

Parity is the property of an integer of whether it is even or odd.

Is 21 an Even Number?


Is 21 an Odd Number?

Yes, the number 21 is an odd number.
Total number of all odd numbers from 1 to 21 is 11
Sum of all the odd numbers from 1 to 21 are 121
The sum of all odd numbers is a perfect square: 121 = 11
An odd number is any integer (a whole number) that cannot be divided by 2 evenly. Odd numbers are the opposite of even numbers.
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Ban number

In recreational mathematics, a ban number is a number that does not contain a particular letter when spelled out in English; in other words, the letter is "banned".
The spelling of 21 in words is "twenty-one", meaning that:
21 is an aban number (a number without the letter a)
21 is not an eban number (as it contains the letter e)
21 is an iban number (a number without the letter i)
21 is not an oban number (as it contains the letter o)
21 is not a tban number (as it contains the letter t)
21 is an uban number (a number without the letter u)
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Numeral systems

How to write 21 in other number systems?
Bengali numerals২১
Eastern Arabic numerals٢١
Hieroglyphs numeralsused in Ancient Egypt𓎇𓏺
Khmer numerals២១
Japanese numerals二十一
Roman numeralsXXI
Thai numerals๒๑
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How do you say 21 in 38 different languages?
Arabicواحد و عشرون
Croatiandvadeset i jedan
Czechdvacet jeden
Danish en og tyve
Estonianblaeve vɔ ɖekɛ
Filipinodalawáng pû’t isá
Frenchvingt et un
Greekείκοσι ένα
Hebrewעשרים ואחת
Icelandictuttugu og einn
Indonesiandua puluh satu
Latviandivdesmit viens
Lithuaniandvidešimt vienas
Persianبیست و یک
Polishdwadzieścia jeden
Portuguesevinte e um
Romaniandouăzeci şi unu
Russianдвадцать один
Serbianдвадесет и један
Slovenedvaset ena
Spanish veintiuno
Swahiliishirini na moja
Turkishyirmi bir
Ukrainianдвадцять один
Vietnamesehai mươi mốt
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Number 21 reversed12
ASCII Code21
Unicode CharacterU+0015
Unix TimestampThu, 01 Jan 1970 00:00:21 +0000
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