1 (number)
Related subjects Mathematics
1


0 1 2 3 4 5 6 7 8 9 → List of numbers — Integers 0 10 20 30 40 50 60 70 80 90 → 

Cardinal  1 one 
Ordinal  1st first 
Numeral system  unary 
Factorization  1 
Divisors  1 
Roman numeral  I 
Roman numeral (Unicode)  Ⅰ, ⅰ 
Arabic  ١ 
Ge'ez  ፩ 
Bengali  ১ 
Chinese numeral  一 , 壹 
Devanāgarī  १ 
Hebrew  א ( Alef) 
Khmer  ១ 
Thai  ๑ 
prefixes  mono (from Greek) uni (from Latin) 
Binary  1 
Octal  1 
Duodecimal  1 
Hexadecimal  1 
1 (one) is a number, numeral, and the name of the glyph representing that number. It represents a single entity. One is sometimes referred to as unity or unit as an adjective. For example, a line segment of "unit length" is a line segment of length 1.
In mathematics, it may represent:
 historically, the first ordinal number, though some modern conventions use 0 (number) as the first ordinal
 the natural number following 0 and preceding 2, the multiplicative identity of the integers
 the corresponding real number 1, the multiplicative identity of the real and complex numbers
 in abstract algebra, it is the multiplicative identity
Mathematics
For any number x:
 x·1 = 1·x = x (1 is the multiplicative identity)
 x/1 = x (see division)
 x^{1} = x, 1^{x} = 1, and for nonzero x, x^{0} = 1 (see exponentiation)
Using ordinary addition, we have 1 + 1 = 2.
One cannot be used as the base of a positional numeral system; sometimes tallying is referred to as "base 1", since only one mark (the tally) is needed, but this is not a positional notation.
The logarithms base 1 is undefined, since 1^{x}=1 and so has no unique inverse function.
In the real number system, 1 can be represented in two ways as a recurring decimal: as 1.000... and as 0.999... (q.v.).
In the Von Neumann representation of natural numbers, 1 is defined as the set {0}. This set has cardinality 1 and hereditary rank 1. Sets like this with a single element are called singletons.
In Principia Mathematica, 1 is defined as the set of all singletons.
In a multiplicative group or monoid, the identity element is sometimes denoted "1", but "e" (from the German Einheit, unity) is more traditional. However, "1" is especially common for the multiplicative identity of a ring. (Note that this multiplicative identity is also often called "unity".)
One is its own factorial, and its own square and cube (and so on, as 1 × 1 × ... × 1 = 1). One is the first figurate number of every kind, such as triangular number, pentagonal number and centered hexagonal number to name just a few.
Because of the multiplicative identity, if f(x) is a multiplicative function, then f(1) must equal 1.
It is also the first and second numbers in the Fibonacci sequence, and is the first number in many mathematical sequences. As a matter of convention, Sloane's early Handbook of Integer Sequences added an initial 1 to any sequence that didn't already have it, and considered these initial 1's in its lexicographic ordering. Sloane's later Encyclopedia of Integer Sequences and its Web counterpart, the OnLine Encyclopedia of Integer Sequences, ignore initial ones in their lexicographic ordering of sequences, because such initial ones often correspond to trivial cases.
One is the empty product.
One is the smallest positive odd integer.
One is a harmonic divisor number.
One is often the internal representation of the Boolean constant true in computer systems.
One is neither a prime number nor a composite number, but a unit, like 1 and, in the Gaussian integers, [imaginary uniti]] and i. The fundamental theorem of arithmetic guarantees unique factorization over the integers only up to units (e.g. 4 = 2^{2 = (1)4×123×22).}
One was formerly considered prime by some mathematicians, using the definition that a prime is divisible only by one and itself. However, this complicates the fundamental theorem of arithmetic, so modern definitions exclude units. The last professional mathematician to publicly label 1 a prime number was Henri Lebesgue in 1899.
One is one of three possible values of the Möbius function: it takes the value one for squarefree integers with an even number of distinct prime factors.
One is the only odd number in the range of Euler's totient function φ(x), in the cases x = 1 and x = 2.
One is the only 1perfect number (see multiply perfect number).
By definition, 1 is the magnitude or absolute value of a unit vector and a unit matrix (more usually called an identity matrix). Note that the term unit matrix is usually used to mean something quite different.
One is the most common leading digit in many sets of data, a consequence of Benford's law.
List of basic calculations
Multiplication  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  50  100  1000  

1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  50  100  1000 
Division  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  

1  0.5  0.25  0.2  0.125  0.1  
1  2  3  4  5  6  7  8  9  10  11  12  13  14  15 
Exponentiation  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  

1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  
1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20 
Evolution of the glyph
The glyph used today in the Western world to represent the number 1, a vertical line, often with a serif at the top and sometimes a short horizontal line at the bottom, traces its roots back to the Indians, who wrote 1 as a horizontal line (in Chinese today this is the way it is written). The Gupta wrote it as a curved line, and the Nagari sometimes added a small circle on the left (rotated a quarter turn to the right, this 9lookalike became the present day numeral 1 in the Gujarati and Punjabi scripts). The Nepali also rotated it to the right, but kept the circle small. This eventually became the top serif in the modern numeral, but the occasional short horizontal line at the bottom probably originates from similarity with the Roman numeral I. In some European countries (e.g., Germany) the little serif at the top is sometimes extended into a long upstroke, sometimes as long as the vertical line, which can lead to confusion with the glyph for seven in other countries. Where the 1 is written with a long upstroke, the number 7 has a horizontal stroke through the vertical line.
While the shape of the 1 character has an ascender in most modern typefaces, in typefaces with text figures the character usually is of xheight, as, for example, in .