Theorem versus Proposition - MathOverflow

Theorem versus Proposition

2010-03-16T10:23:54Z

As a non-native English speaker (and writer) I always had the problem of understanding the distinction between a 'Theorem' and a 'Proposition'. When writing papers, I tend to name only the main result(s) 'Theorem', any auxiliary result leading to this Theorem a 'Lemma' (and, sometimes, small observations that are necessary to prove a Lemma are labeled as 'Claim'). I avoid using the term 'Proposition'.

However, sometimes a paper consists of a number important results (which by all means earn to be called 'Theorem') that are combined to obtain a certain main result. Hence, another term such as 'Proposition' might come in handy, yet I don't know whether it suits either the main or the intermediate results.

So, my question is: When to use 'Theorem' and when to use 'Proposition' in a paper?

Answer by anton for Theorem versus Proposition

2010-03-16T10:55:04Z

The way I do it is this: main results are theorems, smaller results are called propositions. A Lemma is a technical intermediate step which has no standing as an independent result. Lemmas are only used to chop big proofs into handy pieces.

Answer by David Jordan for Theorem versus Proposition

2010-03-16T12:55:08Z

Of course, this is a very subjective question, but I would tend to use "Theorem" only for a statement which has genuine content (whether my own, or one I am citing) and which I wouldn't expect the reader to be able to prove themselves fairly easily. Usually a paper shouldn't have many of these, probably no more than one per section.

"Proposition" I would use after having given a definition, when showing that some fairly straightforward (but not completely obvious) consequence holds; for instance showing that some linear subspace of functions is actually a subalgebra. This is probably close to how you said you use "claim", although I suppose the difference is that you can propose something somewhat out of the blue following a definition, while "claim" is usually directly related to some logical structure which is already moving forward, say to highlight a point midway through the proof of a theorem.

So I make the distinction that Proposition is something that the reader, if so inclined, could easily prove for themselves once they understand the definition. It highlights a result that could just as well have been stated in plain text, emphasizing that while it may be straightforward to prove, it is nevertheless worthy of note.

Answer by JBL for Theorem versus Proposition

2010-03-16T14:31:52Z

I broadly agree with Anton Deitmar, except that I think "Lemma" is difficult to classify (and I tend to just avoid using them). For example, minor results of more generality than the larger theorem they're being used for are frequently lemmas, are they not?

Typically, one doesn't use "Claim" in the same way as "Lemma," "Proposition" or "Theorem" -- I would use it as a sub-heading within the proof of some bigger result, but not as a freestanding result. I use "Proposition" as my default, and "Theorem" for the most important results (e.g., the culmination of some long line of reasoning, the main result in a paper, etc.).

My only complaint with David Jordan's answer is that there are many results that don't have "genuine content" and yet which cannot be easily proven from a definition.

Answer by SB for Theorem versus Proposition

2010-03-16T14:42:04Z

Here is a good rule of thumb:

If you are proud of a result, call it a Theorem. If not, it is a Proposition.

Answer by Pace Nielsen for Theorem versus Proposition

2010-03-16T15:28:18Z

I have seen "Proposition" used for a difficult result that is cited (without proof) from the literature, but which is central to the arguments of the paper. Theorems are usually those big results which the authors of the paper prove themselves. I don't know how prevalent this practice is though.

Answer by Andrea Ferretti for Theorem versus Proposition

2010-03-16T16:03:29Z

I use, like many others, Proposition by default and Theorem for the main results. But I always (I think) cite other people's results as Theorem.

Answer by Jim Humphreys for Theorem versus Proposition

2010-03-16T18:01:49Z

There is no stylebook in mathematics dictating which term to use in which situation, as I think the earlier comments reflect. Every proved statement (even a corollary) might be labelled "theorem", but no one wants to go that far. For me a "lemma" is a technical step in a proof of something bigger, isolated for convenience and possibly for later use. (Unless the "lemma" acquires a life of its own, graduating to "Lemma".) A "theorem" means to me a major result, perhaps the goal of an entire paper. The use of "proposition" is most subjective, but it gets tedious to read a paper containing numerous secondary results claiming to be theorems. Even "corollary" is somewhat subjective, since it might follow instantly from an earlier result or else require other inputs and/or some cleverness to derive. In German there is "Satz" but also "Theorem" to confuse translators.

Answer by Gustav vonChristiaan for Theorem versus Proposition

2011-09-05T04:33:20Z

The trouble with non-quantifiable terms is that said terms are used in a wide variety of areas so they acquire different connotations and as such are "soft" words. One such term is the word "proposition." In Logic it is loosely synonymous with the word statement. E.g "The A-proposition is a Universal Affirmative "proposition" or statement; All humans are mortal beings. Christian Goldbach's statement about "prime #'s has been called his "Theorem." The term "proposition" does imply, indeed, a theory which is rather "tentative" than one we could rightly called "theorem."

Answer by Greg Martin for Theorem versus Proposition

2011-09-05T06:00:45Z

Not that I think I have the definitive answer on this question, but:

Suppose I'm writing a reasonably long paper, broken up into sections. I use Lemmas for technical statements and so on, as many other answerers do. I would rather use Propositions, however, when the result is more global in scale: if it's going to be used outside its section, for example, I'll call it a Proposition, but if it's only used to build up things inside its own section I'll call it a Lemma. Then the Theorems are the statements I want people to take away from the paper.

In this paradigm, a Proposition is more like a Super-Lemma than a Mini-Theorem. I doubt that's a universal sentiment - probably it's not even universal among my own papers....

Answer by Konstantinos Kavalieratos for Theorem versus Proposition

2013-02-24T21:31:02Z

Every proved statement is a theorem and in mathematics there are only axioms definitions and theorems.And of course initial concepts like the concept of a set that can not have strict definition.There is ambiguity in whether a theorem is lemma or proposition or corollary. In LANDAU'S Differential and integral calculus there are only theorems.