Definitions

# Roman letters used in mathematics

Many Roman letters, both capital and small, are used in mathematics, science and engineering to denote by convention specific or abstracted constants, variables of a certain type, units, multipliers, physical entities. Certain letters, when combined with special formatting, take on special meaning.

Below is an alphabetical list of the letters of the alphabet with some of their uses. The field in which the convention applies is mathematics unless otherwise noted.

## Aa

• A represents:
• the first corner of a triangle
• the digit "10" in hexadecimal and other positional numeral systems with a radix of 11 or greater
• the unit ampere for electrical current
• area
• $mathbb\left\{A\right\}$ represents the algebraic numbers or affine space in Algebraic Geometry
• a represents:
• the first side of a triangle (opposite corner A)
• the acceleration in mechanics equations
• the x-intercept of a line
• the unit are for area (100 m²)
• the unit prefix atto (10−18)
• the first term in a sequence or series (eg. Sn = n(a+l)/2)

## Bb

• B represents:
• the digit "11" in hexadecimal and other positional numeral systems with a radix of 12 or greater
• the second corner of a triangle
• a ball (also denoted by $mathcal\left\{B\right\}$ or $mathbb\left\{B\right\}$)
• a basis of a vector space or of a filter (both also denoted by $mathcal\left\{B\right\}$)
• B with various subscripts represents several variations of Brun's constant and Betti numbers
• b represents:
• the second side of a triangle (opposite corner B)
• the y-intercept of a line
• (usually with an index, sometimes with an arrow over it) a basis vector

## Cc

• C represents:
• the third corner of a triangle
• the digit "12" in hexadecimal and other positional numeral systems with a radix of 13 or greater
• the unit coulomb of electrical charge
• C with indices denotes the number of combinations, a binomial coefficient
• $mathbb\left\{C\right\}$ represents the set of complex numbers
• A vertically elongated C with an integer subscript n sometimes denotes the n-th coefficient of a formal power series.
• c represents:
• the third side of a triangle (opposite corner C)
• the unit prefix centi (10−2)
• c represents:
• Small bold C denotes the cardinality of the set of real numbers (the "continuum"), or,

equivalently, of the power set of natural numbers

## Dd

• D represents the digit "13" in hexadecimal and other positional numeral systems with a radix of 14 or greater
• d represents
• the differential operator
• the unit day of time (86 400 s)
• the difference in an arithmetic sequence (eg. Sn = n(2a+(n-1)d)/2)

## Ee

• E represents:
• the digit "14" in hexadecimal and other positional numeral systems with a radix of 15 or greater
• an exponent in decimal numbers 1.2E3 is 1.2×10³ or 1200
• the set of edges in a graph or matroid
• the unit prefix exa (1018)
• Energy in physics
• e represents:
• Euler's number, a transcendental number equal to 2.71828182845… which is used as the base for natural logarithms
• a vector of unit length, especially in the direction of one of the coordinates axes
• the elementary charge in physics

## Hh

• H represents:
• h represents:
• a small increment in the argument of a function
• the unit hour for time (3600 s)
• the Planck constant (6.624 608 96(33) × 10-34 J·s)
• the unit prefix hecto (10²)
• $mathbb\left\{H\right\}$ represents the quaternions (after William Rowan Hamilton, representing the rationals)
• $mathcal\left\{H\right\}$ represents the Hamiltonian in Hamiltonian mechanics

## Ii

• I represents:
• the closed unit interval, which contains all real numbers from 0 to 1, inclusive
• the identity matrix
• i represents:
• the imaginary unit, a complex number that is the square root of −1
• a subscript to denote the ith term (that is, a general term or index) in a sequence or list
• the index to the elements of a vector, written as a subscript after the vector name
• the index to the rows of a matrix, written as the first subscript after the matrix name
• an index of summation using the sigma notation

## Jj

• J represents the unit joule of energy
• j represents:
• the index to the columns of a matrix, written as the second subscript after the matrix name
• in electrical engineering, the square root of −1, instead of i
• in electrical engineering, the principal cube root of 1: $-frac\left\{1\right\}\left\{2\right\}+frac\left\{1\right\}\left\{2\right\}i sqrt 3$

## Ll

• L represents:
• the unit litre of volume
• the space of all integrable real (or complex) functions
• the space of linear maps, as in L(E,F) or L(E) = End(E)
• the Likelihood function
• l represents:
• the length of a side of a rectangle or a rectangular prism (eg. V = lwh; A = lw)
• the last term of a sequence or series (eg. Sn = n(a+l)/2)
• $mathcal\left\{L\right\}$ (or sometimes just L) represents the Lagrangian

## Mm

• M represents:
• m represents:
• the number of rows in a matrix
• the slope in a linear regression or in any line
• the mass in mechanics equations
• the unit metre of length
• the unit prefix milli (10−3)

## Nn

• N represents
• NA represents the Avogadro constant which is the number of entities in one mole (used mainly in the counting of molecules and atoms)
• $mathbb\left\{N\right\}$ represents the natural numbers
• n represents
• the number of columns in a matrix
• the "number of" in algebraic equations.
• the unit prefix nano (10−9)
• the nth term of a sequence or series (eg. tn = a+(n-1)d)

## Oo

• O represents the order of asymptotic behavior of a function; see Big O notation
• O represents $\left(0,0,ldots,0\right)$ — the origin of the coordinate system in Cartesian coordinates

## Pp

• P represents:
• the pressure in physics equations
• the unit prefix peta (1015)
• $mathbb\left\{P\right\}$ represents
• p represents the unit prefix pico (10−12)

## Qq

• $mathbb\left\{Q\right\}$ represents the rational numbers

## Rr

• R represents:
• $mathbb\left\{R\right\}$ represents the set of real numbers and various algebraic structures built upon the set of real numbers, such as $mathbb\left\{R\right\}^n$
• r represents:
• the radius of a circle or sphere
• the ratio of a geometric series (eg. arn-1)

## Ss

• S represents
• s represents:
• an arclength
• the distance traveled in mechanics equations
• the unit second of time
• $mathcal\left\{S\right\}$ represents a system's action in physics

## Tt

• T represents:
• t represents:
• time in graphs, functions or equations
• a term in a sequence or series (eg. tn = tn-1+5)

## Vv

• V represents:
• volume
• the unit volt of voltage
• the set of vertices in a graph
• v represents the velocity in mechanics equations

## Ww

• W represents the unit watt of power

## Yy

• Y represents:
• the unit prefix yotta (1024)
• y represents:
• the unit prefix yocto (10−24)
• y represents:
• a second unknown variable
• the coordinate on the second or vertical axis (backward axis in three dimensions) in a linear coordinate system, or in the viewport of a graph or window in computer graphics