It seems to us […] that mathematics has now reached the stage where formalisation within some particular axiomatic set theory is irrelevant even for foundational studies. It should be possible to specify conditions on a mathematical theory which would sufffice for embeddability within ZF (suplemented by additional axioms of infinity if necessary), but which do not otherwise restrict the possible constructions in that theory. Of course the conditions would apply to ZF itself, and to other possible theories that have been proposed as suitable foundations for mathematics (certain theories of categories, etc.), but would not restrict us to any particular theory. This appendix is in fact a cry for a Mathematicians' Liberation Movement !

Among the permissible kinds of construction we should have:

1. Objects may be created from earlier objects in any reasonably constructive fashion.
2. Equality among the created objects can be any desired equivalence relation.

[…] I hope it is clear that this proposal is not of any particular theory as an alternative to ZF (such as a theory of categories, or of numbers or games considered in this book). What is proposed is instead that we give ourselves the freedom to create arbitrary mathematical theories of these kinds, but prove a metatheorem which ensures once and for all that any such theory could be formalised in terms of any of the standard foundational theories.

The situation is analogous to the theory of vector spaces. Once upon a time these were collections of n-tuples of numbers, and the interesting theorems were those that remained invariant under linear transformations of thes numbers. Now even the initial definitions are invariant, and vector spaces are defined by axioms rather than as particular objects. However, it is proved that every vector space has a base, so that the new theory is much the same as the old. But now no particular base is distinguished, and usually arguments which use particular bases are cumbrous and inelegant compared to arguments directly in terms of the axioms.

We believe that mathematics itself can be founded in an invairant way, which would be equivalent to, but would not involve, formalisation within some theory like ZF. No particular axiomatic theory like ZF would be needed, and indeed attempts to force arbitrary theories into a single formal strait-jacket will probably continue to produce unnecessarily cumbrous and inelegant contortions.

John H. Conway, in the Appendix to Part Zero of On Numbers and Games.

The following is all hideously out of date (I start from relations, not functions, and have been doing so for years now) but I haven't yet finished plundering it for fragments to re-use. Review, 1998/Nov: only the rhetoric isn't done better elsewhere – and that mostly because I now don't do it (as much) elsewhere ;^>

# Notational Foundations

Rather than trying to use layout-markup to mimic the weird and wonderful things mathematicians are used to being able to write, given the joyous liberty of pencil and paper, chalk on a black-board or the like, I have chosen the unorthodox discipline of writing plaintext mathematics. This means abandoning many notational constructs which are essential to standard notation: consequently, it involves such a radical re-invention of notation that I have not been afraid of departures from orthodoxy. Where practical, I have endeavoured to retain as much as possible of the spirit of familiar notations. This has been somewhat helped by taking inspiration from computer programming languages where I have abandoned some more familiar denotations. None the less, I have not been afraid to sacrifice familiarity to maintain conceptual integrity and consistency.

Notation
So how do I propose to write mathematics without the help of all those tools to have which Don Knuth (whose opinion I respect) reckoned it worth writing TeX ?
Names
using more than one letter; character entities and Capitalisation; the consequent need for a visible symbol for multiplication.
Functions
prospering without arrows; sets of functions; function-images and -preimages of sets; bulk actions; mirror dream.
Motivation, Justification and Revolutionary Rhetoric
Plaintext mathematics is all I can offer you: so I'd better invent a notation well-suited to it.
HTML MATH
Why I no longer use it; how to make sense of what I wrote when I still imagined it would happen; the influence of TeX on this.

## Notation

In designing notation, I have drawn much inspiration from the world of computer programming. For automatic programming (or FORmula TRANslation) to be invented, someone had to address the problem of expressing mathematical formulae in a medium far less expressive than the lecturer's writing board. Once the problem had been addressed, some good solutions were found: more (occasionally even better) have leaked out of the woodwork at a steady rate ever since. The astute reader may well recognise echoes of Algol 68, Ponder, Haskell and even the Unix shell in the tools that appear on this site; and maybe even of hint of Larry Wall's rhetoric. Thus language designers borrowed from mathematics; now a mathematician borrows from computer languages to design a notation for mathematics; and the cycle continues, as I have used some of my notation in designing a query language for an object-oriented database …

### Names

In most mathematical notation, values, entities, functions, domains and any manner of other things are named by single letters (with a few exceptions, such as sin, cos, log, exp, det, trace). In order to enlarge the space of available names, writing a letter in (sufficiently) different fonts may allow it to be used as several distinguishable names. Change of case (between upper and lower) is usually also significant. Even so, one runs short of unbound names to play with – especially given the variety of symbols which have specific meanings in contexts which usually apply: π being the most universal. Many fields conflict with one another on symbols (try considering e during discourses on group theory, differential equations and electrodynamics: stop if you get confused); some use different symbols for the same thing (ask for the square root of −1 and you can get either i or j, depending who you ask).

In the world of computer language design it was recognised, early on, that restriction to single-letter names was an unbearable burden. If nothing else, code is much more maintainable if each variable's name reflects the rôle it is to play. Lacking the huge diversity of fonts, which made the single-letter name-space large enough for the job, programmers adopted multi-letter names. This made it much easier to achieve mnemonic naming: sufficiently so that some languages have thrown away even case sensitivity (so that Null, NULL, null and even nuLl are all the same name).

I propose to work with names of arbitrary length. Please ignore however many instances of the empty name there are between anything else: they're just part of the zero-point background of text. I shall typically use single-letter names for dummy variables, though even these will sometimes be longer. Very few glossary entries will be for single-letter names (when, eventually, I get round to putting together a glossary).

I shall use some names of form `&name;` where I think the given name would be a good name for an HTML character entity: if you're very lucky, it will actually be an HTML character entity and you might even see the symbol I wanted in its place. In particular, I shall use α, β, …, ω, Α, …, Ω for the letters of the Greek alphabet: so π is the ratio of a circle's circumference to its diameter (in a plane). If I type &implies; you might see ⇒ (roughly =>, a symbol commonly used to denote logical inference) and, if you don't, you'll see the word implies, albeit embedded in some funny punctuation, so hopefully you'll understand what I meant. Similarly for × (an x-shaped symbol commonly used for multiplication, ×), &mapsto; (or ↦ maps to), ∈ (is in), ∀ (for all), ∃ (there exists) and so on.

Where I define, on one page, a term which I want understood on other pages, I shall try to use words which will make sense as the terms defined; I hope my command of English will be both broad enough and familiar to a wide enough audience to succeed in this. It will not be uncommon for such definitions also to introduce some standard operations on participating entities: I shall aim to give these descriptive names. Where I chose to use several words to make up such a name, I shall capitalise each word (except, sometimes, the first) and stick them all together to form the name: e.g. the operator which takes a subset of a vector space and yields its convex hull is called convexHull. You are welcome to think of these names as being insensitive to case and (as in Algol) gaps: I shalln't use convexhull or convex hull to mean anything incompatible with such a reading, and I may well write it as ConvexHull if ever it appears at the start of a sentence.

One great advantage (which, clearly, I must give up) of single-letter naming is that it makes it possible for mathematicians to dispense with a symbol, ×, for multiplication: when two letters appear side-by-side, the quantities they represent are understood to be multiplied together or composed as functions (and this seldom causes any ambiguity). In really ancient pages here, I have done that when working with single-letter names. However, once longer names are involved, it becomes necessary to separate multiplicands from one another. This could simply be done with a space; however, I chose to use something more definite – a dot. This may be a full stop (that which North American anglophones call a period) or a ·, depending on how often I've seen browsers rendering the latter correctly. Note that mathematicians, having dispensed with the need for × to represent usual multiplication (such a commonplace operation, in any case, as to inspire a wish for some less visually intrusive symbol), have long since put × to good use in related (but weightier) contexts: I have kept it aside for those, hence the need for a dot in its stead !

## Functions

This section is way out of date: I start from relations these days. [Since the following preamble is a bit long, I have chosen to provide readers who just want to cut to the definitions with short-cuts to the definitions of my: source/destination denotation and action denotation. ]

Standard mathematical notation uses f: A → B (in which → is supposed to appear as an arrow, pointing to the right) to denote f is a function from A to B. I'll refer to this as the source and destination denotation for f. This may be followed up by an action denotation for f – i.e. a statement of what value, in B, f ascribes to a typical member of A: either saying f(a)= some expression in a or written as f: a ↦ the expression in a (in which ↦ is supposed to be an arrow with a little vertical bar at its start, to distinguish it from the other arrow). To discuss functions in plaintext, I'm going to need both an action denotation and a source and destination denotation for functions.

I prefer the second, arrow-based, action denotation over the more common illustrative f(a)= whatever, for a variety of reasons: most obviously, because (here reversing the arrows, as it matches more closely the notation I actually use) it can be used with an anonymous function: one can use x.x ←x, the anonymous function which squares its argument, without having to introduce a name for it. In any case there are situations where the arrow form is substantially necessary, so I want to be able to use it, preference or no. However, this required an arrow distinguishable from the first; when I embarked on inventing a notation, I only, in practice, had one arrow at my disposal, namely -> (which I never pretended was a very good one). I'm pleased to now have → and ← at my disposal; but, by the time I did, I'd already settled on a notation using only one kind of arrow, so lost interest in exploiting a greater diversity of arrows.

To cope with having only one arrow, I used a bracketed denotation for functions, which describes the function above, f from A to B, as (A|f:B). This said that f gives values to all members of A, and all the values thus given lie in B. It doesn't say that every value in B gets produced: only that B subsumes the set of values produced. This set, {f(a): a in A}, deserves a brief denotation. Conversely, we sometimes wish to talk about functions defined only on a subset of some domain, as (U|f:B) for some subset, U, of A: at least when this subset makes no other appearance, I prefer not to need to name it. In such a case, A's relationship to f is like that of B – only some of it takes part: likewise {f(u):u in U}'s relationship to f is like that of U – all of it takes part.

See also: discussion of bulk actions of binary operators (e.g. summation as a bulk action derived from (pairwise) addition); and an alternative introduction to the following source/destination notation, generalising parts of it, as natural to the discussion of relations.

I've borrowed my use of |, with a little bending, abstraction and generalisation, from Unix, where | denotes feeding what's come from its left to be received by what's on its right.

I used to toy with the idea of going over to writing the value of function, f, given argument, a, as af or (a)f: this fitted better with the picture of a function labelling an arrow from where it starts to where it ends – the function's argument appears at the left, the function takes it rightwards (i.e. along the arrow) to whatever its image is, whence it may well be taken onwards by a further function applied. I'm told this order of function and argument is used by many algebraists and continental European mathematicians: however, the f(a)-style notation is nearly universal among engineers, physicists and anyone else who looks to Newton, rather than Leibnitz, as origin for the infinitesimal calculus. Since physicists are my primarily intended audience, I resisted the urge to follow this mirror dream; ultimately I decided, instead, to reverse the direction of the arrows (and, consequently, the order of tokens in each of the following denotational forms).

### Source and Destination Denotation

I define a basic bracket notation for functions by:

(A|f:B)
is a function, f, from A to (implicitly: some subset of) B;
(A:f:B)
is a function, f, from some subset of A to B;
(|f)
is the set of values to which f assigns a value (the subset of A to which (A:f:B) implicitly alludes);
(|f:B)
is the set of values to which f assigns a value in B, {a: f(a) in B}, which is contained in (|f) – when B is contained in (f|), this can be written (|f|B);
(f|)
is {f(a): a in (|f)}, the set of values f yields;
(A:f|)
is the set of values f takes on being restricted to A – this can also be written as (A|f|) when A is contained in (|f);
(A|f|B)
says that f is a function from (all of) A to all of B – that is, B= {f(a): a in A} (i.e. (A|f:B) is epic); and
(A:f|B)
says that f is an epic function from some subset of A to B.

Wherever all three of A, f and B appear in these, any | can be replaced with a :, losing only the all of information. Where any of A, f, B is only needed in the discourse to fill its hole in the denotation and isn't the only thing remaining between a | and a preceding ( or following ), I'll leave it out. We can always first demote a | to a : in the excluded case in order to permit an elision; the exclusion ensures that (A:f|), (|f:B) et al. remain denotations for the sets mentioned above. Thus (A|f:) means a function, f, from A (to wherever), (A|:B) means an anonymous function from A to B and (:f:) means a function, f, with unspecified domain and range.

When introducing a function to any discourse, rather than saying given A, B and a function f from A to B I'll just say given (A|f:B), omitting any parts redundant to the discussion and maybe qualifying the statement with some adjectives to tell you what kind of function we're dealing with – e.g. given linear (V|g:W).

I can (and will) also refer to {(A|:B)}, meaning the collection of functions from A to B (conventionally, BA – B with a superscript A). Where some adjective, e.g. linear or monic, is applicable, I'll refer to the collection of such functions by thus qualifying the anonymous function inside the curly braces: e.g. {linear (A|:B)} for the collection of linear maps from A to B. One might, indeed, use {linear (|:)} as the collection of linear morphisms in some given context, and I'll use {(|:) in C} for the collection of morphisms of a category (or arrow-world) C. Likewise, {(A::B)} means the collection of functions from subsets of A to B: {smooth (M::N)} will be very important in the discussion of smooth manifolds.

For a function (A|f:B), the sets (|f:D) and (C:f|) have orthodox denotations f−1(D) and f(C), respectively. It should be noted that these contain potential ambiguity: when A and B are the natural numbers and C, D are members (whence, as required, subsets) thereof, f(C) has a perfectly good meaning as the image, under f, of the (single) natural number C which might not be equal to (C:f|)= {f(i): i in C} = {f(0), …, f(C−1)}, so writing this latter also as f(C) isn't helpful ! The same reasoning applies to f−1(D) when f actually has an inverse.

Having said all that, I should point out that it dates from an old form of my denotations; I have since switched directions so that (B:f:A), when it is a mapping, maps members of A to members of B.

### Action Denotation

This eliminates one of our needs for an arrow, so I only need one; I now use ← (because my mappings take right values to left values), to denote the action of a function on a typical argument. Thus (:x.x ←x:) means the function which maps each value to its square. Where everything in the domain of some function is uniquely expressible as k(x), for some expression k and some argument, x, that k accepts, and I have some other expression h which will accept any such x, I'll write (: h(x) ←k(x) :) with the implied meaning – for example, the function ({integers}: n ← 2.n :{evens}), which halves even numbers.

Now, since (B:f|A) or (B:f:A) denotes a function which is called f, there is a sense in which (B:f:A) = f =(B:f|A) – where it's from and to are intrinsically part of what a function is. We can use this to name a function, for example: square = ({reals}: x.x ← x :{reals}). We can also use it as a way of declaring the form of denotation used for a function: by default, (:f:) is taken to be (:f(x)←x:), but situations which use subscripts, superscripts or other esoteric denotations can declare their denotations using this mechanism.

# Motivation, Justification and Revolutionary Rhetoric

My motives for adopting the discipline of plaintext mathematics combine the playful, the pedagogic and the prosaic. Prosaicly: as discussed below, HTML only really enables me to present plaintext pages. Pedagogically: I believe that the learner gains much by the use of an unfamiliar notation (and the teacher is restrained from certain follies to do with what's obvious to one who already knows it), provided it is clear enough and consistently constructed: and I believe any serious discourse should take pains to introduce and define its terms clearly. Playfully: variety is the spice of life, let's try another notation, just to see what happens when we do. It is also fair to say that I find much widely-used notation to be needlessly clumsy or obfuscated: inventing a replacement gives me the opportunity to cure this. If you find that I have not, please be good enough to write a calm and reasoned explanation of the problem (and maybe some pointers on what you think might fix it) and send it to me, eddy@chaos.org.uk. I know I am capable of improvement ;^>

It may be objected that, in using an unorthodox notation, I do a disservice to the reader: those already familiar with the subject matter meet the barrier of unfamiliar notation; while those meeting it for the first time may understand the subject but will subsequently have trouble making sense of orthodox notation, where they meet it. I will counter with the view that it does us good to vary our point of view: a change in notation shows us familiar subject matter in a new light. I also assert that the notation for any scientific field should evolve, both under pressure from within and in response to changes in the notations of other fields: and that this evolution properly requires occasional upheavals. For scientists to be able to evolve notation, without introducing inconsistencies or extraneous complexity, we must be familiar with variation in notation: otherwise, we shall neither learn the pitfalls (into some of which, I imagine, I fall) nor understand the scope for change open to us.

Adherence to notational orthodoxy also carries the peril of neglecting to explain notation: given that I want these pages to be intelligible to folk who know little of the subject matter (so, if you find something hard to follow, I wish to hear from you and understand what the problem was and [if we can identify it] what I can do about it), it is important that I check that I have explained all the notation I use. Of course, in line with my general problem finding time to edit these pages, there will remain gaps and infelicities for a while yet – partly, of course, because my notation is evolving as I go along. It is, after all, a new start: expect significant variation in anything on time-scales comparable with, or longer than, the age of that thing.

Finally, for this section, if what you want is to skim-read it all, you'll have to wait a long time before I bother to edit the pages to cater to you. I aim these pages at those who, like myself, read and think about each word – the reading equivalent of chewing every morsel 32 times. My target audience, in short, is those who are prepared to take time over reading, and thinking about, the text. If you won't listen to anything unless it can be said in ten seconds, you're going to miss out on much that's important in this world. On the other hand, I know that if I can't produce a decent ten(ish)-word summary of a subject, I haven't quite got it tidy in my mind: so sound-bites will happen, in due course – they just aren't among my high priorities.

Mathematicians are used to working with pencil and paper (and other freely mobile line-drawing tools on two-dimensional surfaces), which gives great liberty as to character set and denotational jiggery-pokery (super- and sub-scripts, integral symbols and other squiggles, convoluted fractions, diagrams with arrows, graphs and so on): it should be no surprise that the notation constructed by folk used to such freedom depends on it. Existing notations are well-suited to the traditional medium; however, I am working in another medium. One can persuade a computer to draw everything needed for orthodox notations, but one has to fight with the medium to achieve these results. I prefer to let the medium itself guide how I express myself in it, so that my effort is principally expended in expressing my thoughts, not in fighting the medium. I want a notation which arises naturally from what it's easy to do with the new medium, just as orthodox notation arises naturally from what it's easy to do with the old medium.

When working on a computer and delivering via the internet, the fundamental medium is what's known technically as an octet-stream – i.e. a sequence of characters. In principle (thanks to Unicode) I can type the whole universe of strange symbols from more cultures than I can count; but, to do so in HTML, I need to type a numeric character entity which, when I see it in my text editor, is just some punctuation and numbers, giving me no clue to which random symbol I've just typed. I can see plain ASCII text as what it is; and I can make sense of the mnemonic character entities, so I prefer to use only these.

When I see `&ndash;` in my document, I know what it stands for (a dash character – with roughly the same width as the letter `n`); and it is easy to remember what to type to obtain a dash of this kind. I have to look up the details to know that `&#x210f;` means the h-bar symbol, ℏ, which theoretical physicists use to denote Dirac's constant; and can't see that this is what I've typed, when I look at my document in my plain text editor – I have to use a web browser to see what the symbol is. Indeed, when I need to use that symbol often, I now use XHTML rather than HTML, since XHTML lets me give the character a name that I can remember; in the file's header, I tell XHTML that `&hbar;` means `&#x210f;` so that I can type the former in the text and, thus, be able to read what I type.

Human ingenuity has found ways to persuade octet-streams to do all manner of fancy things, so that the new medium can be manipulated via all manner of abstractions and made to behave as if it were any other medium one wishes. However, I still sit at my key-board typing – generating a sequence of characters, just like that underlying octet-stream. I can wave a mouse around and do fancy things, but I find it far clumsier than any pen on paper; and I can write faster with a key-board than with a pen. Thus – 'though computers enable us to express many other media as octet-streams – I still, as author, work in a medium consisting fundamentally of a sequence of characters. Furthermore, it is much easier for software to manipulate what I have written (whether to check what I have written in search of errors, to extract meaning from it for use in other media, or to enable a search-engine to grasp which bits of it to index) when the meaning is directly evident in the octet-stream the program sees, rather than having to be revealed by software of far greater complexity than the manipulations I actually wanted the program to perform.

In consequence of this, I have accepted the inevitability of radically different denotational methods and, consequently, sweeping changes to notation. Anyone capable of understanding the subject matter should be able to make sense of the notation I'm creating: I aim to be intelligible to a newcomer, unfamiliar with the mathematics under discussion (who, therefore, would have been learning a new notation in any case); and those who already know the mathematics should have less trouble making sense of it all than any newcomer.

## HTML math mode

[With apologies to those who speak English (as opposed to its close relative, USAish) for calling mathematics math: however, USAish is the base language for HTML, not (whatever the authors may say) English.]

In the mid 1990s, when my web-site was new and I was devising notational forms, W3.org managed to discuss putting support for mathematics (and other scientific notations) into HTML: however, in the drive towards enabling HTML authors to be able to specify the appearance of their pages (i.e. layout) in graphic detail, this attempt at enabling folk to express serious content (via markup) fell by the wayside. The original draft of MATH mode in HTML faded away and didn't get to be part of the common functionality one could rely on from browsers. After all, you can always write it in TeX and convert the dvi output to postscript. Pity about the lack of hypertext links, though. By the time XML begat MathML, necessity had brought forth inventions I preferred.

In the early days of my HTML-writing, I cheerfully expected support for MATH to be along real soon now and began writing pages which used it, in anticipation of the day I could get a version of arena (or, subsequently, Amaya) robust enough to show me my pages. You may, consequently, find stray uses of the old MATH mode in some of my early pages, but I began removing it even while I still entertained the delusion that MATH-mode might go forward.

During the browser wars of the late '90s, Microsoft (desperate to illegally protect its application barrier to entry against the prospect of cross-platform applications – Java's write once run anywhere goal) and Netscape added features to their browsers which were willfully incompatible (with one another and with the W3C's specifications) and encouraged web-site authors to use their extensions in preference to (rather than as enhancements of) the W3C's specifications. The resulting pages – optimized for one or another of these competitors – frequently looked positively dreadful in any other browser (whereas: good use of extensions uses the base standards to make a page that works for all browsers, then tweaks it using extensions so as to work better for browsers supporting the extension). To me this amounted to the authors sneering at anyone who wasn't using the same browser as them: which offended me, so I set out to avoid being so rude to my readers. While I shalln't go much out of my way to cater to deficiencies of any browser (no matter how large its market share) which fails to support the W3C's standards, I do intend that my pages should be intelligible to any half-way decent browser. Since MATH-mode wasn't widely supported, use of it violated that intent, so I backed away from it.

That forced me to work in plain text: and, once I'd solved the problem of saying what I mean in this more easily-typed medium, I ceased yearning to be able to put, on my pages, something resembling the notations designed for a freely-moving hand writing on a two-dimensional continuum. Some day I might take the trouble to write versions of some of my pages using MathML – it's beginning to be supported by browsers (Opera, for example, includes support for it from version 9.50) – but it's never going to be a high priority on this site.

My attempts at using the MATH functionality I once expected are mostly limited to one relatively intelligible piece: HTML character entities for symbols. There might almost be some surviving uses of underscore _ and caret ^ to delimit subscripts and superscripts respectively (though I try to avoid subscripts and superscripts). Given that I've seen browsers actually coping with SUB and SUP tags (for which _ and ^ were merely a short-hand), I've abandoned the use of ^ and _ – except that I use ^ as an antisymmetric product operator. As explained above, under naming, I'm still using HTML character entities, though only where I either find the entity name mnemonic or am used to browsers decoding it correctly. Since mid-2005, I've begun using XHTML (despite the risk that some browsers may cope poorly with this newer, although now fairly well-established, standard) to enable pages to use mnemonic character entities and have them displayed suitably.

In using the character entities (in HTML), I've assumed that when a browser meets &burble; it either shows it as such or decodes it and displays the appropriate character. Since the names are at least mnemonic, this should be intelligible, though ugly, even on browsers which don't decode them. So if you see something preceded by &lcub; (left curly brace) and followed by &rcub; (right curly brace), you'll soon learn to recognise it as enclosed in curly braces {i.e., it's a denotation for a collection or set}. Likewise, if you meet something saying ∈, read it as though the words is in appeared in its place. You'll find a fairly full list of the valid symbols in the W3.org tour of HTML 3 and a full list of the ones I know in my list of HTML Character Entities.

Post-script (2004/March): I note that XML+CSS can do a very nice job of rendering mathematics in classical notation. The MathML markup system has now been defined but I'm unimpressed with it, though there's lively debate on its merits as compared to XML+CSS and some folk are toying with the idea of an XHTML+CSS solution. The very fact that they see a need to do that speaks volumes about MathML.

### TeX remains

Where I couldn't find an HTML MATH mode character denotation for a symbol I wanted, I borrowed from TeX or made one up in the same spirit. I still do this to some extent. Thus if you meet `&csname;` and recognise `\csname` from TeX, please read it as the latter. In general, the names used are fairly clear as to what they mean – so ignorance of TeX shouldn't present much trouble; and I'll strive to always introduce them properly. Such bogus character entities are known probably bad HTML and I list them below, without concerning myself over which are borrowed from TeX and which I've invented.

&implies;
implies (roughly =>, a double-shafted rightwards-pointing arrow) or ⇒
&iff;
if and only iff (roughly <=>, a double-shafted double-headed arrow) or ⇔ meaning the statements to either side imply one another.
&mapsto;
maps to (roughly |--> i.e. a right-arrow with a short vertical bar at the left end of its shaft) or ↦
&on;
composition (usually denoted by a slightly raised small o) or ∘
&tensor;
tensor product (a × inscribed within a circle), ⊗ or ⊗
&intersect;
intersection (resembling an upturned U), ∩ or ∩

# Detritus

I'm hanging some bits off here pending plundering them for anything for which I don't have a better home, possibly ditching the rest. The first two mainly just need notational conversions, I think.

A demonstration
of features HTML 3 has at some stage proposed to support for mathematics – you'll find similar (better) in W3.org's guided tour of HTML 3.
Bayesian Inference
an attempt at translating David MacKay's lecture hand-out into HTML 3. This is stalled unfinished.
Written by Eddy.