** Comments about mathematical usage**, extending those in my post on abuse of notation.

Geoffrey Pullum, in his post Dogma vs. Evidence: Singular *They, *makes some good points about usage that I want to write about in connection with *mathematical* usage. There are two different attitudes toward language usage abroad in the English-speaking world. (See Note [1])

- What matters is what people
*actually write and say*. Usage in this sense may often be reported with reference to particular dialects or registers, but in any case it is based on*evidence*, for example citations of quotations or a linguistic corpus. (Note [2].) This approach is scientific. - What matters is what a particular writer (of usage or style books)
*believes*about standards for speaking or writing English. Pullum calls this "faith-based grammar". (People who think in this way often use the word "grammar" for usage.) This approach is unscientific.

People who write about mathematical usage fluctuate between these two camps.

My writings in the Handbook of Mathematical Discourse and abstractmath.org are mostly evidence based, with some comments here and there deprecating certain usages because they are confusing to students. I think that is about the right approach. ** Students need to know what is actual mathematical usage**, even usage that many mathematicians deprecate.

Most math usage that is deprecated (by me and others) is deprecated for a* reason. *This reason should be explained, and that is enough to stop it being faith-based. To make it really scientific you ought to cite evidence that students have been confused by the usage. Math education people have done some work of this sort. Most of it is at the K-12 level, but some have worked with college students observing the way the solve problems or how they understand some concepts, and this work often cites examples.

## Examples of usage to be deprecated

### Powers of functions

$latex f^n(x)$ can mean either iterated composition or multiplication of the values. For example, $latex f^2(x)$ can mean $latex f(x)f(x)$ or $latex f(f(x))$. This is exacerbated by the fact that in undergrad calculus texts, $latex \sin^{-1}x$ refers to the arcsine, and $latex \sin^2 x$ refers to $latex \sin x\sin x$. This causes innumerable students trouble. It is a Big Deal.

### In

Set "in" another set. This is discussed in the Handbook. My impression is that for students the problem is that they confuse "element of" with "subset of", and the fact that "in" is used for both meanings is not the primary culprit. That's because most sets in practice don't have both sets and non-sets as elements. So the problem is a Big Deal when students first meet with the concept of set, but the notational confusion with "in" is only a Small Deal.

### Two

This is not a Big Deal. But I have personally witnessed students (in upper level undergrad courses) that were confused by this.

### Parentheses

The many uses of parentheses, discussed in abstractmath. (The Handbook article on parentheses gives citations, including one in which the notation "$latex (a,b)$" means open interval once and GCD once in the same sentence!) I think the only part that is a Big Deal, or maybe Medium Deal, is the fact that the value of a function $latex f$ at an input $latex x$ can be written either "$latex f\,x$" or as "$latex f(x)$". In fact, we do without the parentheses when the name of the function is a convention, as in $latex \sin x$ or $latex \log x$, and with the parentheses when it is a variable symbol, as in "$latex f(x)$". (But a substantial minority of mathematicians use $latex f\,x$ in the latter case. Not to mention $latex xf$.) This causes some beginning calculus students to think "$latex \sin x$" means "sin" *times *$latex x$*.*

### More

The examples given above are only a sampling of troubles caused by mathematical notation. Many others are mentioned in the Handbook and in Abstractmath, but they are scattered. I welcome suggestions for other examples, particularly at the college and graduate level. Abstractmath will probably have a separate article listing the examples someday…

## Notes

[1] The situation Pullum describes for English is probably different in languages such as Spanish, German and French, which have Academies that dictate usage for the language. On the other hand, from what I know about them most speakers of those languages ignore their dictates.

[2] Actually, they may use more than one corpus, but I didn't want to write "corpuses" or "corpora" because in either way I would get sharp comments from faith-based usage people.

## References on mathematical usage

Bagchi, A. and C. Wells (1997), Communicating Logical Reasoning.

Bagchi, A. and C. Wells (1998) Varieties of Mathematical Prose.

Bullock, J. O. (1994), ‘Literacy in the language of mathematics’. *American Mathematical Monthly, *volume 101, pages 735743.

de Bruijn, N. G. (1994), ‘The mathematical vernacular, a language for mathematics with typed sets’. In *Selected Papers on Automath, *Nederpelt, R. P., J. H. Geuvers, and R. C. de Vrijer, editors, volume 133 of *Studies in Logic and the Foundations of Mathematics, *pages 865 935. ** **

Epp, S. S. (1999), ‘The language of quantification in mathematics instruction’. In *Developing Mathematical Reasoning in Grades K-12. *Stiff, L. V., editor (1999), NCTM Publications. Pages 188197.

Gillman, L. (1987), *Writing Mathematics Well. *Mathematical Association of America

Higham, N. J. (1993), *Handbook of Writing for the Mathematical Sciences. *Society for Industrial and Applied Mathematics.

Knuth, D. E., T. Larrabee, and P. M. Roberts (1989), *Mathematical Writing, *volume 14 of *MAA Notes. *Mathematical Association of America.

Krantz, S. G. (1997), *A Primer of Mathematical Writing. *American Mathematical Society.

O'Halloran, K. L. (2005), *Mathematical Discourse: Language, Symbolism And Visual Images.* Continuum International Publishing Group.

Pimm, D. (1987), *Speaking Mathematically: Communications in Mathematics Classrooms. *Routledge & Kegan Paul.

Schweiger, F. (1994b), ‘Mathematics is a language’. In *Selected Lectures from the 7th International Congress on Mathematical Education, *Robitaille, D. F., D. H. Wheeler, and C. Kieran, editors. Sainte-Foy: Presses de l’Université Laval.

Steenrod, N. E., P. R. Halmos, M. M. Schiffer, and J. A. Dieudonné (1975), *How to Write Mathematics. *American Mathematical Society.

Wells, C. (1995), Communicating Mathematics: Useful Ideas from Computer Science.

Wells, C. (2003), Handbook of Mathematical Discourse

Wells, C. (ongoing), Abstractmath.org.