Editing Coefficient

{{short description|Multiplicative factor in a mathematical expression}}
{{Other uses}}
{{refimprove|date=May 2017}}

In [[mathematics]], a '''coefficient''' is a [[Factor (arithmetic)|multiplicative factor]] involved in some [[Summand|term]] of a [[polynomial]], a [[series (mathematics)|series]], or any other type of [[expression (mathematics)|expression]]. It may be a [[Dimensionless quantity|number without units]], in which case it is known as a '''numerical factor'''.<ref name=80000-1:2009>{{cite web|title=ISO 80000-1:2009|url=https://www.iso.org/standard/30669.html|publisher=[[International Organization for Standardization]]|access-date=2019-09-15}}</ref> It may also be a [[constant (mathematics)|constant]] with [[units of measurement]], in which it is known as a '''constant multiplier'''.<ref name=80000-1:2009/> In general, coefficients may be any [[mathematical expression|expression]] (including [[Variable (mathematics)|variables]] such as {{mvar|a}}, {{mvar|b}} and {{mvar|c}}).<ref>{{Cite web |last=Weisstein |first=Eric W. |title=Coefficient |url=https://mathworld.wolfram.com/Coefficient.html |access-date=2020-08-15 |website=mathworld.wolfram.com |language=en}}</ref><ref name=80000-1:2009/> When the combination of variables and constants is not necessarily involved in a [[product (mathematics)|product]], it may be called a ''[[parameter]]''.<ref name=80000-1:2009/>  
For example, the polynomial <math>2x^2-x+3</math> has coefficients 2, −1, and 3, and the powers of the variable <math>x</math> in the polynomial <math>ax^2+bx+c</math> have coefficient parameters <math>a</math>, <math>b</math>, and <math>c</math>.

A {{vanchor|Constant coefficient|text='''constant coefficient'''}}, also known as '''constant term''' or simply '''constant''', is a quantity either implicitly attached to the [[zeroth power]] of a variable or not attached to other variables in an expression; for example, the constant coefficients of the expressions above are the number 3 and the parameter ''c'', involved in 3{{=}}''c''{{sdot}}''x''<sup>0</sup>. 
The coefficient attached to the highest degree of the variable in a polynomial of one variable is referred to as the '''leading coefficient'''; for example, in the example expressions above, the leading coefficients are 2 and ''a'', respectively.

In the context of [[differential equation]]s, these equations can often be written in terms of polynomials in one or more unknown functions and their derivatives. In such cases, the coefficients of the differential equation are the coefficients of this polynomial, and these may be non-constant functions. A coefficient is a ''constant coefficient'' when it is a [[constant function]]. For avoiding confusion, in this context a coefficient that is not attached to unknown functions or their derivatives is generally called a ''constant term'' rather than a constant coefficient. In particular, in a [[linear differential equation#Non-homogeneous equation with constant coefficients|linear differential equation with constant coefficient]], the constant coefficient term is generally not assumed to be a constant function.

== Terminology and definition ==
In mathematics, a '''coefficient''' is a multiplicative factor in some [[Summand|term]] of a [[polynomial]], a [[series (mathematics)|series]], or any [[expression (mathematics)|expression]].  For example, in the polynomial
<math display="block">7x^2-3xy+1.5+y,</math>
with variables <math>x</math> and <math>y</math>, the first two terms have the coefficients 7 and −3. The third term 1.5 is the constant coefficient. In the final term, the coefficient is 1 and is not explicitly written.

In many scenarios, coefficients are numbers (as is the case for each term of the previous example), although they could be parameters of the problem—or any expression in these parameters. In such a case, one must clearly distinguish between symbols representing variables and symbols representing parameters. Following [[René Descartes]], the variables are often denoted by {{mvar|x}}, {{mvar|y}}, ..., and the parameters by {{mvar|a}}, {{mvar|b}}, {{mvar|c}}, ..., but this is not always the case. For example, if {{mvar|y}} is considered a parameter in the above expression, then the coefficient of {{mvar|x}} would be {{math|−3''y''}}, and the constant coefficient (with respect to {{mvar|x}}) would be {{math|1.5 + ''y''}}.

When one writes 
<math display="block">ax^2+bx+c,</math>
it is generally assumed that {{mvar|x}} is the only variable, and that {{mvar|a}}, {{mvar|b}} and {{mvar|c}} are parameters; thus the constant coefficient is {{mvar|c}} in this case.

{{anchor|leading coefficient}}Any [[polynomial]] in a single variable {{mvar|x}} can be written as
<math display="block">a_k x^k + \dotsb + a_1 x^1 + a_0</math>
for some [[nonnegative integer]] <math>k</math>, where <math>a_k, \dotsc, a_1, a_0</math> are the coefficients.  This includes the possibility that some terms have coefficient 0; for example, in <math>x^3 - 2x + 1</math>, the coefficient of <math>x^2</math> is 0, and the term <math>0x^2</math> does not appear explicitly.  For the largest <math>i</math> such that <math>a_i \ne 0</math> (if any), <math>a_i</math> is called the '''leading coefficient''' of the polynomial. For example, the leading coefficient of the polynomial
<math display="block">4x^5 + x^3 + 2x^2</math>
is 4. This can be generalised to multivariate polynomials with respect to a [[monomial order]], see {{section link|Gröbner basis|Leading term, coefficient and monomial}}.

==Linear algebra==
In [[linear algebra]], a [[system of linear equations]] is frequently represented by its [[coefficient matrix]].  For example, the system of equations
<math display="block">
\begin{cases} 
2x + 3y = 0 \\
5x - 4y = 0
\end{cases},</math>
the associated coefficient matrix is <math>\begin{pmatrix}
2 & 3 \\
5 & -4
\end{pmatrix}.
</math>  Coefficient matrices are used in algorithms such as [[Gaussian elimination]] and [[Cramer's rule]] to find solutions to the system.

The '''leading entry''' (sometimes ''leading coefficient''{{cn|date=March 2022}}) of a row in a matrix is the first nonzero entry in that row. So, for example, in the matrix
<math display="block">
\begin{pmatrix}
1 & 2 & 0 & 6\\
0 & 2 & 9 & 4\\
0 & 0 & 0 & 4\\
0 & 0 & 0 & 0
\end{pmatrix},
</math>
the leading coefficient of the first row is 1; that of the second row is 2; that of the third row is 4, while the last row does not have a leading coefficient.

Though coefficients are frequently viewed as [[constant (mathematics)|constants]] in elementary algebra, they can also be viewed as variables as the context broadens. For example, the [[coordinates]] <math>(x_1, x_2, \dotsc, x_n)</math> of a [[vector (geometric)|vector]] <math>v</math> in a [[vector space]] with [[basis (linear algebra)|basis]] <math>\lbrace e_1, e_2, \dotsc, e_n \rbrace </math> are the coefficients of the basis vectors in the expression 
<math display="block"> v = x_1 e_1 + x_2 e_2 + \dotsb + x_n e_n .</math>

==See also==
*[[Correlation coefficient]]
*[[Degree of a polynomial]]
*[[Monic polynomial]]
*[[Binomial coefficient]]

== References ==
<references />

==Further reading==

*Sabah Al-hadad and C.H. Scott (1979) ''College Algebra with Applications'', page 42, Winthrop Publishers, Cambridge Massachusetts {{ISBN|0-87626-140-3}} .
*Gordon Fuller, Walter L Wilson, Henry C Miller, (1982) ''College Algebra'', 5th edition, page 24, Brooks/Cole Publishing, Monterey California {{ISBN|0-534-01138-1}} .

[[Category:Polynomials]]
[[Category:Mathematical terminology]]
[[Category:Algebra]]
[[Category:Numbers]]
[[Category:Variables (mathematics)]]