# 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 It was superseded in 2009 by ISO 80000-2:2009 and subsequently revised in 2019 as ISO-80000-2:2019.

Its definitions include the following:

## 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 qp. Sometimes → is used.
xA p(x)
(∀xA) p(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)
(∃xA) p(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
xA x belongs to A; x is an element of the set A
xA x does not belong to A; x is not an element of the set A The negation stroke can also be vertical.
Ax the set A contains x (as an element) same meaning as xA
Ax the set A does not contain x (as an element) same meaning as xA
{ } {x1, x2, ..., xn} set with elements x1, x2, ..., xn also {xiiI}, where I denotes a set of indices
{ ∣ } {xAp(x)} set of those elements of A for which the proposition p(x) is true Example: {x$\mathbb {R}$ x > 5}
The ∈A can be dropped where this set is clear from the context.
card card(A) number of elements in A; cardinal of A
AB difference between A and B; A minus B The set of elements which belong to A but not to B.
AB = { xxAxB }
AB can also be used.
the empty set
$\mathbb {N}$ the set of natural numbers; the set of positive integers and zero $\mathbb {N}$ = {0, 1, 2, 3, ...}
Exclusion of zero is denoted by an asterisk:
$\mathbb {N}$ * = {1, 2, 3, ...}
$\mathbb {N}$ k = {0, 1, 2, 3, ..., k − 1}
$\mathbb {Z}$ the set of integers $\mathbb {Z}$ = {..., −3, −2, −1, 0, 1, 2, 3, ...}

$\mathbb {Z}$ * = $\mathbb {Z}$ ∖ {0} = {..., −3, −2, −1, 1, 2, 3, ...}

$\mathbb {Q}$ the set of rational numbers $\mathbb {Q}$ * = $\mathbb {Q}$ ∖ {0}
$\mathbb {R}$ the set of real numbers $\mathbb {R}$ * = $\mathbb {R}$ ∖ {0}
$\mathbb {C}$ the set of complex numbers $\mathbb {C}$ * = $\mathbb {C}$ ∖ {0}
[,] [a,b] closed interval in $\mathbb {R}$ from a (included) to b (included) [a,b] = {x$\mathbb {R}$ axb}
],]
(,]
]a,b]
(a,b]
left half-open interval in $\mathbb {R}$ from a (excluded) to b (included) ]a,b] = {x$\mathbb {R}$ a < xb}
[,[
[,)
[a,b[
[a,b)
right half-open interval in $\mathbb {R}$ from a (included) to b (excluded) [a,b[ = {x$\mathbb {R}$ ax < b}
],[
(,)
]a,b[
(a,b)
open interval in $\mathbb {R}$ from a (excluded) to b (excluded) ]a,b[ = {x$\mathbb {R}$ a < x < b}
BA B is included in A; B is a subset of A Every element of B belongs to A. ⊂ is also used.
BA B 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".
CA C is not included in A; C is not a subset of A ⊄ is also used.
AB A 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".
AC A does not include C (as subset) ⊅ is also used. AC means the same as CA.
AB union of A and B The set of elements which belong to A or to B or to both A and B.
AB = { xxAxB }
$\bigcup _{i=1}^{n}A_{i}$ union of a collection of sets $\bigcup _{i=1}^{n}A_{i}=A_{1}\cup A_{2}\cup \ldots \cup A_{n}$ , the set of elements belonging to at least one of the sets A1, ..., An. $\bigcup {}_{i=1}^{n}$ and $\bigcup _{i\in I}$ , $\bigcup {}_{i\in I}$ are also used, where I denotes a set of indices.
AB intersection of A and B The set of elements which belong to both A and B.
AB = { xxAxB }
$\bigcap _{i=1}^{n}A_{i}$ intersection of a collection of sets $\bigcap _{i=1}^{n}A_{i}=A_{1}\cap A_{2}\cap \ldots \cap A_{n}$ , the set of elements belonging to all sets A1, ..., An. $\bigcap {}_{i=1}^{n}$ and $\bigcap _{i\in I}$ , $\bigcap {}_{i\in I}$ are also used, where I denotes a set of indices.
AB complement of subset B of A The 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.
(,...,) (a1a2, ..., an) ordered n-tuple a1, a2, ..., an⟩ is also used.
× A × B cartesian product of A and B The 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 ΔA set of pairs (a, a) ∈ A × A where aA; diagonal of the set A × A ΔA = { (a, a) ∣ aA }
idA is also used.

## Miscellaneous signs and symbols

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

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

## 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) = −ab

## Functions

Example Meaning and verbal equivalent Remarks
$f:D\rightarrow C$ function f has domain D and codomain C Used to explicitly define the domain and codomain of a function.
$f\left(S\right)$ $\left\{f\left(x\right)\mid x\in S\right\}$ 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...
e$x$ exponential function to the base e of $x$ log$a$ $x$ logarithm to the base $a$ of $x$ lb $x$ binary logarithm (to the base 2) of $x$ lb $x$ = log2$x$ ln $x$ natural logarithm (to the base e) of $x$ ln $x$ = loge$x$ lg $x$ common logarithm (to the base 10) of $x$ lg $x$ = log10$x$ ## Circular and hyperbolic functions

Example Meaning and verbal equivalent Remarks
π ratio of the circumference of a circle to its diameter π = 3.141 59...

## 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 $[xyz]=[xyz];$ $[dxdydz];$ cartesian x1, x2, x3 for the coordinates and e1, e2, e3 for the base vectors are also used. This notation easily generalizes to n-mensional space. ex, ey, ez form an orthonormal right-handed system. For the base vectors, i, j, k are also used.
ρ, φ, z $[x,y,z]=[\rho \cos(\phi ),\rho \sin(\phi ),z]$ 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 $[x,y,z]=r[\sin(\theta )\cos(\phi ),\sin(\theta )\sin(\phi ),\cos(\theta )]$ 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
${\vec {a}}$ 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) ...