# What is the difference between an indeterminate and variable?

From Wikipedia-

In mathematics, and particularly in formal algebra, an indeterminate is a symbol that is treated as a variable, but does not stand for anything else but itself and is used as a placeholder in objects such as polynomials and formal power series.

So, how it is different from variable which is also used as a placeholder? What is the use of the indeterminate?

$x^2 + 3x + 3$ is a polynomial which has $x$ as indeterminate. The $x$ doesn't stand for any particular number, it's just the indeterminate of the expression. You can ask various questions about the polynomial (e.g. "does it factor?"), but you can't ask "what is $x$?".

On the other hand, you might try to solve the equation $x^2 + 3x + 3 = 7$. Here $x$ is being used as a variable: it stands for an unknown number which makes the equation true when substituted for $x$.

• I like this answer. Saying that a variable is something that you can ask the question "what is $x$?" to doesn't strike me as a perfect characterization, but maybe it's a good starting intuition. – 6005 Sep 15 '16 at 5:42

An indeterminate is a syntactic tool that is used to write down expressions in a certain way. If we are considering polynomials, for example, we may use $x$ as an indeterminate to write them down easily: $1 + 4x - x^4$. If $x$ is an indeterminate, then this is just a way that we can write down the list of coefficients $(1, 4, 0, 0, -1)$ more easily. We use the indeterminate $x$ for stylistic reasons and to ease computation; formally speaking, we are really talking about the list of coefficients.

On the other hand, a variable is used to represent an element of some set. Consider these examples: "Let $x$ be an integer." "For all integers $n$, $n$ is even or odd." "Let $x = 37$." In all cases, the variable stands for an element of a set, whether it be a specific element or an arbitrary element. Variables are what we use to go about mathematical reasoning. Variables are fundamental to writing proofs; we cannot prove something about all elements of a set or any for all or there exists statement without utilizing variables.

Of course, usually when we use an indeterminate (at least, in the case of formal power series and polynomials) we want to emphasize a sort of analogy with variables, because it is possible to "plug in" a specific value for that indeterminate in the same way we can plug in a specific value to a variable. Therefore, in some ways indeterminates just stand for a particular kind of variable (but not the other way around).

• I am not trying to disagree, but I think this really has to do with what you've been taught rather than explicitly what these two things mean. I don't think they have explicit definitions in the sense that, in my mind a parameter, an indeterminate and a variable are all the same thing; it's just in what subject you encounter the terminology the most seems to be what people will say each should be used for and derive their meaning with that context in mind. – Faraad Armwood Sep 15 '16 at 5:34
• @FaraadArmwood I largely agree with you. If you like, you can think of "parameter" and "indeterminate" and even "constant" as just meaning "variable". But there are only some contexts in which we would refer to something as an indeterminate, and there is a formal difference because with an indeterminate variable, the variable is not taken to be an element of any set and isn't quantified over existentially or universally. – 6005 Sep 15 '16 at 5:36
• Well said. With that in mind and wanting to have clarify when referring to different objects, redefining things by context is probably better anyway. I like your answer, I just wanted to make sure I didn't have the wrong idea. – Faraad Armwood Sep 15 '16 at 5:39
• @6005 Wait. How can we think of a "constant" as just meaning "variable". It puts the whole calculus to fallacy. Isn't it? – ankit Sep 15 '16 at 14:00
• @6005 I smell polynumbers here, am I right? – Ubi hatt Jan 4 at 16:51