A coefficient is a multiplicative factor in some term of a polynomial, a series or any expression; it is usually a number, but may be any expression.
Often coefficients are numbers, 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 Descartes, the variables are often denoted by x, y, ..., and the parameters by a, b, c, ..., but it is not always the case. For example, if y is considered as a parameter in the above expression, the coefficient of x is −3y, and the constant coefficient is 1.5 + y.
When one writes
- $ ax^2+bx+c $,
it is generally supposed that x is the only variable and that a, b and c are parameters; thus the constant coefficient is c in this case.
Similarly, any polynomial in one variable x can be written as
- $ a_k x^k + \dotsb + a_1 x^1 + a_0 $
for some positive integer $ k $, where $ a_k, \dotsc, a_1, a_0 $ are coefficients.
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