# Which letter of the alphabet is least often used as a mathematical variable? **URL:** \ **Category:** Factual Questions\ **Created:** [July 26, 2019, 2:52am UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651 "2019-07-26T02:52:06Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [July 26, 2019, 5:19pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/21 "2019-07-26T17:19:47Z")
> [@MikeS](#): > > In upper-level mathematics, _e_ is commonly used to denote the identity element of a group. But in that case, it also isn’t really a variable, though. The other usage I can think of is in denoting unit vectors—_e[sub]x[/sub]_, _e[sub]y[/sub]_, and _e[sub]z[/sub]_, for example. But that’s also not really a variable. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 26, 2019, 5:43pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/22 "2019-07-26T17:43:05Z")
I suppose that the Greek letters that look exactly like Latin letters are probably the least common, because why would you use the Greek one? So omicron is probably the least-used. And I once graded a student who used Cyrillic letters for his auxiliary variables (he was from somewhere in Eastern Europe). It did guarantee that none of his variables were already in use in the problem, but it took me a while to realize that ш and щ were two different letters. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 26, 2019, 5:49pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/23 "2019-07-26T17:49:21Z")
For Greek letters, I expect the omicron to be the least used, for the same reason o is rarely used among the Latin letters. ETA: ninja’d [curse you Chronos] ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [July 26, 2019, 6:01pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/24 "2019-07-26T18:01:17Z")
> [@Chronos](#): > > And I once graded a student who used Cyrillic letters for his auxiliary variables (he was from somewhere in Eastern Europe). It did guarantee that none of his variables were already in use in the problem, but it took me a while to realize that ш and щ were two different letters. Cyrillic probably would take the cake for most confusing variable if somebody were to use З (ze), though. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [July 26, 2019, 8:10pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/25 "2019-07-26T20:10:32Z")
> [@Half\_Man\_Half\_Wit](#): > > But in that case, it also isn’t really a variable, though. Sure it is. It’s a placeholder to whatever the identity value is. It might stand for 1, it might stand for 0, it might stand for +∞, etc. You’re stating a theorem in Group Theory and it goes “blah, blah, blah, _e_, blah, blah, blah.” _e_ here serves the same function as a _x_ does in Real Analysis where a theorem states “blah, blah, blah, _x_, blah, blah, blah.” This is unlike, say, in Computer Science where _e_ or something like it\* refers to the empty string. An empty string is a fixed value. A string of length zero. _That_ is a constant. - I prefer λ myself. ---
**Author:** ![F.U.Shakespeare](https://avatars.discourse-cdn.com/v4/letter/f/aca169/32.png) [@F.U.Shakespeare](https://boards.straightdope.com/u/F.U.Shakespeare)\ **Post date:** [July 27, 2019, 2:22am UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/26 "2019-07-27T02:22:59Z")
> [@Chronos](#): > > If we’re getting into the Greek alphabet, lower-case **zeta** and xi are very seldom used, **mostly because they’re difficult to draw correctly.** But I’ve seen them show up as indices in general relativity, occasionally. And I like to use them, because I’m a bit proud of how well I can draw them. One of my engineering professors referred to zeta as “the Greek letter squiggly”. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [July 27, 2019, 3:15am UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/27 "2019-07-27T03:15:07Z")
> [@F.U.Shakespeare](#): > > One of my engineering professors referred to zeta as “the Greek letter squiggly”. It’s no more squiggly than a (handwritten) Latin z, is it? At least not in my handwriting. Also, letters are not “drawn”— they are _written_ (at least when they’re not typed, printed, etc.) Besides the famous ζ-function, I want to say I recall very typical uses along the lines of: let ζ be a primitive nth root of unity, and consider the cyclotomic field **Q** (ζ)… ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [July 27, 2019, 12:16pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/28 "2019-07-27T12:16:59Z")
I was looking at the properties of [2019 OK](https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2019OK;cad=1)\* yesterday and noticed another use of _e_ as a variable: eccentricity of orbit. Even for a given object, _e_ is not a fixed value. For example the eccentricity of Mars changes over time a surprising amount. And 2019 OK’s eccentricity had to have gotten a big change when it passed us by 0.19 lunar distances the other day. Another use in Physics is good old _e_ = _mc_[sup]2[/sup]. Two out of the 3 are variables. It’s also used as the symbol for electron charge but that’s a constant. Oddly, _m_ is also used for the mass of an electron. Figuring out _e_/_m_ was a notable early Physics measurement. (I did this measurement in E&M lab in college. Um, that’s Electricity and Magnetism, resp.) - Happening to get the sequence label “OK” for an asteroid that passed this close catches the eye. It’s impact energy would have been 30 Hiroshima A-bombs. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 27, 2019, 1:47pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/29 "2019-07-27T13:47:17Z")
When it stands for energy, E is generally capitalized. Likewise for when it’s electric field. You’re right, though, that eccentricity is usually lower-case e. m for mass is pretty typical. What it’s the mass of is generally made clear by context. In finding the e/m ratio, the context is electrons, but in a problem about a baseball, it’s probably the mass of a baseball. **ftg** , the empty string isn’t any more a constant than the identity element of an arithmetic is. In fact, the empty string is itself the identity element of concatenation. But the empty string in C isn’t the same as the empty string in Fortran isn’t the same as the empty string in Java, and an empty Unicode string isn’t the same as an empty ASCII string, and so on. But once you’ve set the context, the empty string (or the identity element) is well-defined and unique. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [July 27, 2019, 1:53pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/30 "2019-07-27T13:53:52Z")
> [@scr4](#): > > Probably **e** , because it almost represents the mathematical constant **e** (2.71828…), not a variable. i is another good candidate for similar reasons: it’s the square root of -1 ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [July 27, 2019, 4:05pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/31 "2019-07-27T16:05:36Z")
> [@PoppaSan](#): > > No love for j? Word on the street is that Ladies Love Cool J. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 27, 2019, 4:14pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/32 "2019-07-27T16:14:57Z")
But i is one of the standard default variable names in any sort of discreet mathematics, like as an index for an array. And, as mentioned, it often gets used for current (physicists usually capitalize it for current, but engineers might or might not). ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [July 27, 2019, 4:19pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/33 "2019-07-27T16:19:47Z")
> [@Chronos](#): > > But i is one of the standard default variable names in any sort of discreet mathematics Shh, don’t tell! ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [July 27, 2019, 4:20pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/34 "2019-07-27T16:20:35Z")
> [@Chronos](#): > > **ftg** , the empty string isn’t any more a constant than the identity element of an arithmetic is. In fact, the empty string is itself the identity element of concatenation. But the empty string in C isn’t the same as the empty string in Fortran isn’t the same as the empty string in Java, and an empty Unicode string isn’t the same as an empty ASCII string, and so on. But once you’ve set the context, the empty string (or the identity element) is well-defined and unique. Those are _implementations_ of a constant. Not the constant itself. You don’t think of π as a non-constant just because it is represented in different ways on a computer do you? I must say I am amazed you made this statement. It is silly. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [July 27, 2019, 6:11pm UTC](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651/35 "2019-07-27T18:11:34Z")
I suspect it is o. I don’t think I ever used it and I use all the others all the time. When I use l, though, it is not an ordinary l, but one with a loop like a handwritten l. One thing I would like to clarify is that mathematical journals no longer distinguish typographically between constants and variables. Once upon a time, constants were set upright and variables in italics. But typesetters can’t tell what is intended. Nowadays (and this is strongly enforced by the journal I am TeX editor for) single letter identifiers are set in italics (specifically, math italics which differ somewhat from text italics), while multicharacter identifiers such as sin, log, and the like are set upright. So _sin_ is just the product s_i_n, while sin is just the standard trigonometric function. For Greek letters, needless to say, the only capitals used are those that differ from the roman capitals. How could you tell a Rho from a P? All the lower case Greek letters are common, again except omicron. The least used capital is probably Xi (which consists of three horizontal lines), although I have seen it and even used it. I don’t find either xi or zeta hard to write or to recognize. I am told that Russian mathematicians use roman and greek alphabets for their identifiers, but not usually cyrillic. I once heard a talk by the Russian Yuri Matiyasevich talking about the equation he had discovered in connection with his solution to Hilbert’s 10th problem. The first unsolvable equation he had discovered had something like 32 variables. Finally, he had found one with only 26 and he was happy because that meant that he could write it using only the roman alphabet (so one of them had to be o). [Previous page](https://boards.straightdope.com/t/which-letter-of-the-alphabet-is-least-often-used-as-a-mathematical-variable/837651.md?page=1)