# ISO 31-11

ISO 31-11:1992 was the part of international standard ISO 31 that defines mathematical signs and symbols for use in physical sciences and technology. It was superseded in 2009 by ISO 80000-2:2009 and subsequently revised in 2019 as ISO-80000-2:2019.[1]

Its definitions include the following:[2]

## Mathematical logic

Sign Example Name Meaning and verbal equivalent Remarks
pq conjunction sign p and q
pq disjunction sign p or q (or both)
¬ ¬ p negation sign negation of p; not p; non p
pq implication sign if p then q; p implies q Can also be written as q ⇐ p. Sometimes → is used.
xA p(x)
(∀xAp(x)
universal quantifier for every x belonging to A, the proposition p(x) is true The "A" can be dropped where A is clear from context.
xA p(x)
(∃xAp(x)
existential quantifier there exists an x belonging to A for which the proposition p(x) is true The "A" can be dropped where A is clear from context.
∃! is used where exactly one x exists for which p(x) is true.

## Sets

Sign Example Meaning and verbal equivalent Remarks
xAx belongs to A; x is an element of the set A
xAx does not belong to A; x is not an element of the set AThe negation stroke can also be vertical.
Axthe set A contains x (as an element)same meaning as xA
Axthe set A does not contain x (as an element)same meaning as xA
{ }{x1, x2, ..., xn}set with elements x1, x2, ..., xnalso {xi | iI}, where I denotes a set of indices
{ | }{xA | p(x)}set of those elements of A for which the proposition p(x) is trueExample: {x ∈ ℝ | x > 5}
The A can be dropped where this set is clear from the context.
cardcard(A)number of elements in A; cardinal of A
AB difference between A and B; A minus BThe set of elements which belong to A but not to B.
AB = { x | xAxB }
AB
can also be used.
the empty set
the set of natural numbers; the set of positive integers and zeroℕ = {0, 1, 2, 3, ...}
Exclusion of zero is denoted by an asterisk:
* = {1, 2, 3, ...}
k = {0, 1, 2, 3, ..., k − 1}
the set of integersℤ = {..., −3, −2, −1, 0, 1, 2, 3, ...}
* = ℤ ∖ {0} = {..., −3, −2, −1, 1, 2, 3, ...}
the set of rational numbers* = ℚ ∖ {0}
the set of real numbers* = ℝ ∖ {0}
the set of complex numbers* = ℂ ∖ {0}
[,][a,b]closed interval in ℝ from a (included) to b (included)[a,b] = {x ∈ ℝ | axb}
],]
(,]
]a,b]
(a,b]
left half-open interval in ℝ from a (excluded) to b (included)]a,b] = {x ∈ ℝ | a < xb}
[,[
[,)
[a,b[
[a,b)
right half-open interval in ℝ from a (included) to b (excluded)[a,b[ = {x ∈ ℝ | ax < b}
],[
(,)
]a,b[
(a,b)
open interval in ℝ from a (excluded) to b (excluded)]a,b[ = {x ∈ ℝ | a < x < b}
BAB is included in A; B is a subset of AEvery element of B belongs to A. ⊂ is also used.
BAB is properly included in A; B is a proper subset of A Every element of B belongs to A, but B is not equal to A. If ⊂ is used for "included", then ⊊ should be used for "properly included".
CAC is not included in A; C is not a subset of A⊄ is also used.
ABA includes B (as subset)A contains every element of B. ⊃ is also used. BA means the same as AB.
AB.A includes B properly.A contains every element of B, but A is not equal to B. If ⊃ is used for "includes", then ⊋ should be used for "includes properly".
ACA does not include C (as subset)⊅ is also used. AC means the same as CA.
ABunion of A and BThe set of elements which belong to A or to B or to both A and B.
AB = { x | xAxB }
${\displaystyle \bigcup _{i=1}^{n}A_{i}}$union of a collection of sets${\displaystyle \bigcup _{i=1}^{n}A_{i}=A_{1}\cup A_{2}\cup \ldots \cup A_{n}}$, the set of elements belonging to at least one of the sets A1, ..., An. ${\displaystyle \bigcup {}_{i=1}^{n}}$ and ${\displaystyle \bigcup _{i\in I}}$, ${\displaystyle \bigcup {}_{i\in I}}$ are also used, where I denotes a set of indices.
ABintersection of A and BThe set of elements which belong to both A and B.
AB = { x | xAxB }
${\displaystyle \bigcap _{i=1}^{n}A_{i}}$intersection of a collection of sets${\displaystyle \bigcap _{i=1}^{n}A_{i}=A_{1}\cap A_{2}\cap \ldots \cap A_{n}}$, the set of elements belonging to all sets A1, ..., An. ${\displaystyle \bigcap {}_{i=1}^{n}}$ and ${\displaystyle \bigcap _{i\in I}}$, ${\displaystyle \bigcap {}_{i\in I}}$ are also used, where I denotes a set of indices.
ABcomplement of subset B of AThe set of those elements of A which do not belong to the subset B. The symbol A is often omitted if the set A is clear from context. Also AB = AB.
(,)(a, b)ordered pair a, b; couple a, b(a, b) = (c, d) if and only if a = c and b = d.
a, b is also used.
(,...,)(a1, a2, ..., an)ordered n-tuplea1, a2, ..., an is also used.
× A × Bcartesian product of A and BThe set of ordered pairs (a, b) such that aA and bB.
A × B = { (a, b) | aAbB }
A × A × ⋯ × A is denoted by An, where n is the number of factors in the product.
Δ ΔAset of pairs (a, a) ∈ A × A where aA; diagonal of the set A × AΔA = { (a, a) | aA }
idA
is also used.

## Miscellaneous signs and symbols

Sign Example Meaning and verbal equivalent Remarks
HTML TeX
${\displaystyle {\stackrel {\mathrm {def} }{=}}}$ ${\displaystyle a\ {\stackrel {\mathrm {def} }{=}}\ b}$ a is by definition equal to b [2] := is also used
= ${\displaystyle =}$ a = b a equals b ≡ may be used to emphasize that a particular equality is an identity.
${\displaystyle \neq }$ ab a is not equal to b ${\displaystyle a\not \equiv b}$ may be used to emphasize that a is not identically equal to b.
${\displaystyle {\stackrel {\wedge }{=}}}$ ${\displaystyle a\ {\stackrel {\wedge }{=}}\ b}$ a corresponds to b On a 1:106 map: ${\displaystyle 1{\text{ cm }}{\stackrel {\wedge }{=}}\ 10{\text{ km}}}$.
${\displaystyle \approx }$ ab a is approximately equal to b The symbol ≃ is reserved for "is asymptotically equal to".

${\displaystyle {\begin{matrix}\sim \\\propto \end{matrix}}}$ ab
ab
a is proportional to b
< ${\displaystyle <}$ a < b a is less than b
> ${\displaystyle >}$ a > b a is greater than b
${\displaystyle \leq }$ ab a is less than or equal to b The symbol ≦ is also used.
${\displaystyle \geq }$ ab a is greater than or equal to b The symbol ≧ is also used.
${\displaystyle \ll }$ ab a is much less than b
${\displaystyle \gg }$ ab a is much greater than b
${\displaystyle \infty }$ infinity
()
[]
{}
⟨⟩
${\displaystyle {\begin{matrix}()\\{[]}\\\{\}\\\langle \rangle \end{matrix}}}$ ${\displaystyle {\begin{matrix}{(a+b)c}\\{[a+b]c}\\{\{a+b\}c}\\{\langle a+b\rangle c}\end{matrix}}}$ ac + bc, parentheses
ac + bc, square brackets
ac + bc, braces
ac + bc, angle brackets
In ordinary algebra, the sequence of ${\displaystyle (),[],\{\},\langle \rangle }$ in order of nesting is not standardized. Special uses are made of ${\displaystyle (),[],\{\},\langle \rangle }$ in particular fields.
${\displaystyle \|}$ AB ∥ CD the line AB is parallel to the line CD
${\displaystyle \perp }$ AB ⊥ CD the line AB is perpendicular to the line CD[3]

## Operations

Sign Example Meaning and verbal equivalent Remarks
+ a + b a plus b
ab a minus b
± a ± b a plus or minus b
ab a minus or plus b −(a ± b) = −a ∓ b

## Functions

Example Meaning and verbal equivalent Remarks
f : DCfunction f has domain D and codomain C Used to explicitly define the domain and codomain of a function.
f(S){ f(x) | xS }Set of all possible outputs in the codomain when given inputs from S, a subset of the domain of f.

## Exponential and logarithmic functions

Example Meaning and verbal equivalent Remarks
e base of natural logarithms e = 2.718 28...
ex exponential function to the base e of x
logax logarithm to the base a of x
lb x binary logarithm (to the base 2) of x lb x = log2x
ln x natural logarithm (to the base e) of x ln x = logex
lg x common logarithm (to the base 10) of x lg x = log10x

## Circular and hyperbolic functions

Example Meaning and verbal equivalent Remarks
π ratio of the circumference of a circle to its diameter π ≈ 3.14159

## Complex numbers

Example Meaning and verbal equivalent Remarks
i, j imaginary unit; i2 = −1 In electrotechnology, j is generally used.
Re z real part of z z = x + iy, where x = Re z and y = Im z
Im z imaginary part of z
|z| absolute value of z; modulus of z mod z is also used
arg z argument of z; phase of z z = reiφ, where r = |z| and φ = arg z, i.e. Re z = r cos φ and Im z = r sin φ
z* (complex) conjugate of z sometimes a bar above z is used instead of z*
sgn z signum z sgn z = z / |z| = exp(i arg z) for z ≠ 0, sgn 0 = 0

## Matrices

Example Meaning and verbal equivalent Remarks
A matrix A

## Coordinate systems

Coordinates Position vector and its differential Name of coordinate system Remarks
x, y, z [x y z]; [dx dy dz] cartesian x1, x2, x3 for the coordinates and e1, e2, e3 for the base vectors are also used. This notation easily generalizes to n-dimensional space. ex, ey, ez form an orthonormal right-handed system. For the base vectors, i, j, k are also used.
ρ, φ, z [x, y, z] = [ρ cos(φ), ρ sin(φ), z] cylindrical eρ(φ), eφ(φ), ez form an orthonormal right-handed system. lf z = 0, then ρ and φ are the polar coordinates.
r, θ, φ [x, y, z] = r[sin(θ)cos(φ), sin(θ)sin(φ), cos(θ)] spherical er(θ,φ), eθ(θ,φ),eφ(φ) form an orthonormal right-handed system.

## Vectors and tensors

Example Meaning and verbal equivalent Remarks
a
${\displaystyle {\vec {a}}}$
vector a Instead of italic boldface, vectors can also be indicated by an arrow above the letter symbol. Any vector a can be multiplied by a scalar k, i.e. ka.

## Special functions

Example Meaning and verbal equivalent Remarks
Jl(x) cylindrical Bessel functions (of the first kind) ...