Variable (mathematics)

Early history The earliest uses of an "unknown quantity" date back to at least the Ancient Egyptians with the Moscow Mathematical Papyrus (c. 1500 BC) which described problems with unknowns rhetorically, called the "Aha problems". The "Aha problems" involve finding unknown quantities (referred to as aha, "stack") if the sum of the quantity and part(s) of it are given (The Rhind Mathematical Papyrus also contains four of these types of problems). For example, problem 19 asks one to calculate a quantity taken times and added to 4 to make 10. In modern mathematical notation: . Around the same time in Mesopotamia, mathematics of the Old Babylonian period (c. 2000 BC – 1500 BC) was more advanced, also studying quadratic and cubic equations. Diophantus of Alexandria, pioneered a form of syncopated algebra in his Arithmetica (c. 200 AD), which introduced symbolic manipulation of expressions with unknowns and powers, but without modern symbols for relations (such as equality or inequality) or exponents. An unknown number was called \zeta. The square of \zeta was \Delta^v; the cube was K^v; the fourth power was \Delta^v\Delta; and the fifth power was \Delta K^v. So for example, what would be written in modern notation as: x^3 - 2x^2 + 10x -1, would be written in Diophantus's syncopated notation as: : \Kappa^{\upsilon} \overline{\alpha} \; \zeta \overline{\iota} \;\, \pitchfork \;\, \Delta^{\upsilon} \overline{\beta} \; \Mu \overline{\alpha} \,\; In the 7th century BC, Brahmagupta used different colours to represent the unknowns in algebraic equations in the Brāhmasphuṭasiddhānta. One section of this book is called "Equations of Several Colours". Greek and other ancient mathematical advances, were often trapped in long periods of stagnation, and so there were few revolutions in notation, but this began to change by the early modern period. Early modern period At the end of the 16th century, François Viète introduced the idea of representing known and unknown numbers by letters, nowadays called variables, and the idea of computing with them as if they were numbers—in order to obtain the result by a simple replacement. Viète's convention was to use consonants for known values, and vowels for unknowns. In 1637, René Descartes "invented the convention of representing unknowns in equations by , , and , and knowns by , , and ". Contrarily to Viète's convention, Descartes' is still commonly in use. The history of the letter x in math was discussed in an 1887 Scientific American article. Starting in the 1660s, Isaac Newton and Gottfried Wilhelm Leibniz independently developed the infinitesimal calculus, which essentially consists of studying how an infinitesimal variation of a time-varying quantity, called a Fluent, induces a corresponding variation of another quantity which is a function of the first variable. Almost a century later, Leonhard Euler fixed the terminology of infinitesimal calculus, and introduced the notation for a function , its variable and its value . Until the end of the 19th century, the word variable referred almost exclusively to the arguments and the values of functions. In the second half of the 19th century, it appeared that the foundation of infinitesimal calculus was not formalized enough to deal with apparent paradoxes such as a nowhere differentiable continuous function. To solve this problem, Karl Weierstrass introduced a new formalism consisting of replacing the intuitive notion of limit by a formal definition. The older notion of limit was "when the variable varies and tends toward , then tends toward ", without any accurate definition of "tends". Weierstrass replaced this sentence by the formula : (\forall \epsilon >0) (\exists \eta >0) (\forall x) \;|x-a| \; \Rightarrow |L-f(x)| in which none of the five variables is considered as varying. This static formulation led to the modern notion of variable, which is simply a symbol representing a mathematical object that either is unknown, or may be replaced by any element of a given set (e.g., the set of real numbers). == Notation ==

Variables are generally denoted by a single letter, most often from the Latin alphabet and less often from the Greek, which may be lowercase or capitalized. The letter may be followed by a subscript: a number (as in ), another variable (), a word or abbreviation of a word as a label () or a mathematical expression (). Under the influence of computer science, some variable names in pure mathematics consist of several letters and digits. Following René Descartes (1596–1650), letters at the beginning of the alphabet such as , , are commonly used for known values and parameters, and letters at the end of the alphabet such as , , are commonly used for unknowns and variables of functions. In printed mathematics, the norm is to set variables and constants in an italic typeface. For example, a general quadratic function is conventionally written as , where , and are parameters (also called constants, because they are constant functions), while is the variable of the function. A more explicit way to denote this function is , which clarifies the function-argument status of and the constant status of , and . Since occurs in a term that is a constant function of , it is called the constant term. Specific branches and applications of mathematics have specific naming conventions for variables. Variables with similar roles or meanings are often assigned consecutive letters or the same letter with different subscripts. For example, the three axes in 3D coordinate space are conventionally called , , and . In physics, the names of variables are largely determined by the physical quantity they describe, but various naming conventions exist. A convention often followed in probability and statistics is to use , , for the names of random variables, keeping , , for variables representing corresponding better-defined values. Conventional variable names • , , , (sometimes extended to , ) for parameters or coefficients • , , , ... for situations where distinct letters are inconvenient • or for the th term of a sequence or the th coefficient of a series • , , for functions (as in ) • , , (sometimes or ) for varying integers or indices in an indexed family, or unit vectors • and for the length and width of a figure • also for a line, or in number theory for a prime number not equal to • (with as a second choice) for a fixed integer, such as a count of objects or the degree of a polynomial • for a prime number or a probability • for a prime power or a quotient • for a radius, a remainder or a correlation coefficient • for time • , , for the three Cartesian coordinates of a point in Euclidean geometry or the corresponding axes • for a complex number, or in statistics a normal random variable • , , , , for angle measures • (with as a second choice) for an arbitrarily small positive number • for an eigenvalue • (uppercase sigma) for a sum, or (lowercase sigma) in statistics for the standard deviation • for a mean == Specific kinds of variables ==

It is common for variables to play different roles in the same mathematical formula, and names or qualifiers have been introduced to distinguish them. For example, the general cubic equation : ax^3+bx^2+cx+d=0, is interpreted as having five variables: four, , which are taken to be given numbers and the fifth variable, is understood to be an unknown number. To distinguish them, the variable is called an unknown, and the other variables are called parameters or coefficients, or sometimes constants, although this last terminology is incorrect for an equation, and should be reserved for the function defined by the left-hand side of this equation. In the context of functions, the term variable refers commonly to the arguments of the functions. This is typically the case in sentences like "function of a real variable", " is the variable of the function ", " is a function of the variable " (meaning that the argument of the function is referred to by the variable ). In the same context, variables that are independent of define constant functions and are therefore called constant. For example, a constant of integration is an arbitrary constant function that is added to a particular antiderivative to obtain the other antiderivatives. Because of the strong relationship between polynomials and polynomial functions, the term "constant" is often used to denote the coefficients of a polynomial, which are constant functions of the indeterminates. Other specific names for variables are: • An unknown is a variable in an equation which has to be solved for. • An indeterminate is a symbol, commonly called variable, that appears in a polynomial or a formal power series. Formally speaking, an indeterminate is not a variable, but a constant in the polynomial ring or the ring of formal power series. However, because of the strong relationship between polynomials or power series and the functions that they define, many authors consider indeterminates as a special kind of variables. • A parameter is a quantity (usually a number) which is a part of the input of a problem, and remains constant during the whole solution of this problem. For example, in mechanics the mass and the size of a solid body are parameters for the study of its movement. In computer science, parameter has a different meaning and denotes an argument of a function. • Free variables and bound variables • A random variable is a kind of variable that is used in probability theory and its applications. All these denominations of variables are of semantic nature, and the way of computing with them (syntax) is the same for all. Dependent and independent variables In calculus and its application to physics and other sciences, it is rather common to consider a variable, say , whose possible values depend on the value of another variable, say . In mathematical terms, the dependent variable represents the value of a function of . To simplify formulas, it is often useful to use the same symbol for the dependent variable and the function mapping onto . For example, the state of a physical system depends on measurable quantities such as the pressure, the temperature, the spatial position, ..., and all these quantities vary when the system evolves, that is, they are function of the time. In the formulas describing the system, these quantities are represented by variables which are dependent on the time, and thus considered implicitly as functions of the time. Therefore, in a formula, a dependent variable is a variable that is implicitly a function of another (or several other) variables. An independent variable is a variable that is not dependent. The property of a variable to be dependent or independent depends often of the point of view and is not intrinsic. For example, in the notation , the three variables may be all independent and the notation represents a function of three variables. On the other hand, if and depend on (are dependent variables) then the notation represents a function of the single independent variable . Examples If one defines a function from the real numbers to the real numbers by : f(x) = x^2+\sin(x+4) then x is a variable standing for the argument of the function being defined, which can be any real number. In the identity : \sum_{i=1}^n i = \frac{n^2+n}2 the variable is a summation variable which designates in turn each of the integers (it is also called index because its variation is over a discrete set of values) while is a parameter (it does not vary within the formula). In the theory of polynomials, a polynomial of degree 2 is generally denoted as , where , and are called coefficients (they are assumed to be fixed, i.e., parameters of the problem considered) while is called a variable. When studying this polynomial for its polynomial function this stands for the function argument. When studying the polynomial as an object in itself, is taken to be an indeterminate, and would often be written with a capital letter instead to indicate this status. Example: the ideal gas law Consider the equation describing the ideal gas law, PV = Nk_\text{B}T. This equation would generally be interpreted to have four variables, and one constant. The constant is , the Boltzmann constant. One of the variables, , the number of particles, is a positive integer (and therefore a discrete variable), while the other three, , and , for pressure, volume and temperature, are continuous variables. One could rearrange this equation to obtain as a function of the other variables, P(V, N, T) = \frac{Nk_\text{B}T}{V}. Then , as a function of the other variables, is the dependent variable, while its arguments, , and , are independent variables. One could approach this function more formally and think about its domain and range: in function notation, here is a function P: \mathbb{R}_{>0} \times \mathbb{N} \times \mathbb{R}_{>0} \rightarrow \mathbb{R}. However, in an experiment, in order to determine the dependence of pressure on a single one of the independent variables, it is necessary to fix all but one of the variables, say . This gives a function P(T) = \frac{Nk_\text{B}T}{V}, where now and are also regarded as constants. Mathematically, this constitutes a partial application of the earlier function . This illustrates how independent variables and constants are largely dependent on the point of view taken. One could even regard as a variable to obtain a function P(V, N, T, k_\text{B}) = \frac{Nk_\text{B}T}{V}. == Moduli spaces ==

Considering constants and variables can lead to the concept of moduli spaces. For illustration, consider the equation for a parabola, y = ax^2 + bx + c, where , , , and are all considered to be real. The set of points in the 2D plane satisfying this equation trace out the graph of a parabola. Here, , and are regarded as constants, which specify the parabola, while and are variables. Then instead regarding , and as variables, we observe that each set of 3-tuples corresponds to a different parabola. That is, they specify coordinates on the 'space of parabolas': this is known as a moduli space of parabolas. == See also ==