Talk:Function (mathematics)

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	Mathematics grading:	B Class	Top Importance	Field: Basics
A vital article.

Could you archive talk, rather than just cutting it out? Charles Matthews 12:04, 7 Nov 2004 (UTC)

See Talk:Function (mathematics)/archive.

Archive 1 to May 2006

1 function notation
2 Non-Numeric Functions
3 notation for functions.
- 3.1 Standard notation
  - 3.1.1 Thank you very much
4 warning to editors
5 first function definition
6 a vital article should be good for reference
7 Merge?
8 Definition of a function
9 Bijection, injection and surjection redirects here

[edit] function notation

In my fairly short university career I have be taught that if you define a function as f(x)=x^2, for example, then you can also say f=x^2. Similarly if you say y=x^2 then you can also say y(x)=x^2. i.e. There is no difference between f(x) and f they mean the same thing but with slightly different stresses on what the function is used for.

What I am suggesting is that this should be be flagged as different notation "f(x)=y" saying that instead "f(x)=f" or "y=y(x)".

The classic notation is y = f(x). In the past twenty years, I see just f and also y(x) more and more frequently, and see no problem as long as these notations are used unambiguously. If they are mentioned in the article, it should be mentioned that these are modernisms. Rick Norwood 14:13, 27 May 2006 (UTC)

Study of foundations and use of computers has forced more careful use of notation, though convenient brief forms persist. Contrast

$f\colon \mathbb{C} \rarr \reals \,\!$

$x \mapsto x \bar{x} \,\!$

with

$f(x) = x\bar{x} \,\!$

$y = x\bar{x} . \,\!$

More provocatively, contrast with Standard ML's

fun f x = x * conj x

or the C programming language

float f(struct complex x) {return ctimes(x,cconj(x));}

or the Common Lisp

(lambda (x) (* x (conjugate x)))

In the SML case type polymorphism broadens the domain and range possibilities, while in the C case the result is restricted to a float rather than, say, a double. The Lisp case does not name the function, nor does it restrict the domain and range. --KSmrq^T 18:45, 27 May 2006 (UTC)

I have never seen f=x². I have seen that f(x) is used instead of f when speaking of the function (although f(x) is the value at x), usually it is clear from the context that x is "unbound" i.e. not a particular value, but a "placeholder". But it is clear that the information f(x)=x² is incomplete insofar we cannot know on what set the function is defined (N, R, C ?) and even less if it is surjective or so (not surjective on N, although this is smaller than R₊, surjective on C although this is larger than R).

Other notations not mentioned here are things like $E\ni x \mapsto^{\!\!\!\!\!\!f}~ y=f(x)\in F$ , which some mathematicians like. (We understood that WP is not for mathematicians; a pity since it could have been useful.)

Also, the C language example is quite mathematical (float as well as double are sets and ctimes is a function (with precise domain and codomain) both well-defined in the most strict mathematical sense), while the ML one is not, neither the Lisp: these clearly are not functions in the set-theoretical sense, (though maybe in the category theory sense, if it is understood that arguments are in the category that has the operations occuring in the function body, and results are in the category implied by the involved functions/operators).

Maple or MuPAD allow both, writing f:= x -> x*x ; or (with or without type specification), f:=proc(x :: float)::float x*x end; — MFH:Talk 15:10, 13 October 2006 (UTC)

[edit] Non-Numeric Functions

"A function need not involve numbers. An example of a function that does not use numbers is the function that assigns to each nation its capital. In this case Capital(France) = Paris."

I'm not a mathematician, but I question the validity of this. In the example given, the fucntion, Capital(x) = y does not yield exactly one output per input. Capital(France) = Vichy and Capital(France) = Aachen are arguably valid outputs for the input "France." Generally a nation has only one capital (at a time). However it's misleading to equate the mathematical certainty of a fuction (exactly one output per input) with a historical rule-of-thumb (unless a war has disrupted the normal political process, a nation, during modern times, shall usually have only one capital). Perhaps someone could come up with a better example of a Non-Numeric Function? Thanks! 66.17.118.207 15:11, 26 June 2006 (UTC)

Absolute non-ambiguity is impossible. Even f(x) = x + 5, which seems non-ambiguous, would admit a question whether the addition took place in the natural number system, in the real number system, or in the integers modulo 7. I suspect that any example of a non-numerical function would suffer from similar problems. One has to use common sense. Rick Norwood 15:37, 26 June 2006 (UTC)

Are you serious? First, this is a non-mathematical example. Second, France has only one capital now. Third, if some country happens to have simultaneous multiple capitals we can simply allow the function to return a set. Fourth, if we really need to accommodate different facts at different times we have tools such as modal logic. Finally, are you serious?! --KSmrq^T 23:49, 26 June 2006 (UTC)

Well just as with mathmatical functions you could say that the complete function should include the domain (i.e. year = 2006)DougBrown 16:39, 2 November 2006 (UTC)

[edit] notation for functions.

See Talk:Fourier_transform#Where_to_put_it. The conventional notation for functions, where you write f(x) meaning f, is insufficient when it is not obvious what the independent variable is. Please comment on the suggestion for a solution. Bo Jacoby 11:44, 7 August 2006 (UTC)

I think what this article does, begin with the simple case and go on to more advanced cases, is the only way to go. Another advanced problem is free variables vs. bound variables. Probably worth discussing, but only near the end of the article.

A discussion of independent and dependent variables should be discussed in the context of implicit functions. Rick Norwood 13:27, 7 August 2006 (UTC)

Bo, a simple request: provide a mainstream citation showing that your proposed notation (e.g. f = (x^y←x) instead of f(x) = x^y) is an established notation in mathematics. When you were asked for citations for this notation in Talk:Fourier transform, the only thing you provided was a loosely similar syntax in the J programming language. (I've seen ← used to denote functions, and I've seen "=", but I don't recall seeing this kind of combination of "=" and ←; it seems obscure at best.)

Wikipedia should stick with the conventional notations for a given topic; more obscure/uncommon ones can be mentioned but should be presented as such. Notations not used in mainstream mathematics publications should not be included in Wikipedia at all, according to policy.

—Steven G. Johnson 05:36, 11 August 2006 (UTC)

Steven, I appreciate your entering a discussion rather than to delete my contribution. You are slowly growing civilized.

The use of the colon in the notation, f $\;:\;$ x $\mapsto{}$ y is unconventional and pointless and confusing. Equality should always be written with the equality sign: f=x→y. Parentheses should be used to fix the order of operations: f=(x→y).

The arrow points from the independent to the dependent variable. Usually an asymmetric symbol signifying an asymmetric connection is reversed to mean the reverse connection. For example, (a<b) is the same thing as (b>a). Both notations are free to be used. So (x→y) has the same meaning as (y←x), that x is mapped to y and that y is a function of x.

The lambda notation, λ x.y, uses the symmetrical symbol (.) to signify an asymmetrical connection between independent and dependent variable. This makes it hard to read. Using the arrow instead of the dot is an improvement: λ x→y . Now the lambda is no longer needed and we are left with x→y.

The convention that letter x implicitely signifies the independent variable, is useful for simple cases, but insufficient for complicated cases. An explicite indication of the independent variable is needed to distinguish the power function x^y from the exponential function x^y.

You know the equality sign, the arrow and the parentheses. The combination f=(y←x) means what it is supposed to mean. Even if you don't recall seeing this combination of digits: 1329685, you will not request me to provide a mainstream citation showing that it is an established notation in mathematic.

Bo Jacoby 10:20, 11 August 2006 (UTC)

Standard mathematical notation has its share of confusions and inconsistencies. Innovative notations may claim to be far superior. That's all irrelevant here. We're documenting, not advocating.

Besides that, the claims above exhibit their own confusion. If the lambda notation shouldn't use a "symmetrical symbol" then we'd better replace the minus sign in x−y as well. But only for those who cannot distinguish left from right.

The notation f $\;:\;$ x $\mapsto{}$ y is non-standard and incorrect, and not what the article should use. The standard is exhibited in a previous comment on this talk page.

$f\colon \mathbb{C} \to \reals \,\!$

$x \mapsto \frac{1}{2}(x+\bar{x}) \,\!$

On the first line, the colon–arrow notation gives the name, domain, and codomain of the function, using a simple arrow (" $\to\,\!$ "). On the second line, the function name is not mentioned; instead pure function notation is used, with a different arrow symbol (" $\mapsto\,\!$ "). I'll fix this in the article. (I thought I already had; mea culpa.)

It is absolutely true that other notation shows up. In commutative diagrams the colon–arrow notation is altered by putting the function name above the arrow. Occasionally in mathematics, and in (reverse Polish notation) programming languages like Forth, function names are written after their arguments. Omission of parentheses is common, so long as there is no risk of confusion. And so on. What we need to do is collect some of the major mathematical variants in a section on notations, or even split out a separate article if the list gets long.

What we do not need is to clutter the main presentation.

All editors should take note that the topic of this article is familiar to almost anyone. That tends to cause problems, in two ways. First, such an article will be consulted by readers with an enormous range of sophistication, from pre-college to post-doctorate. Second, the editors will also have a diverse range, and will likely have seen more computer notation than pure mathematics. Even more than usual it will help to be kind and thoughtful in what we say to our readers. We also need to be extra aware of our fellow editors' possible limitations, and of our own. --KSmrq^T 23:32, 11 August 2006 (UTC)

If we're documenting, not advocating, the confusions and inconsistencies of standard mathematical notation, then we must tell the embarrassing truth: "There is no useful standard notation for the solution f to the equation y=f(x). The power function f defined by the equation f(x)=x^y is not the same thing as the exponential function f defined by the equation f(y)=x^y. The arrow notation y←x is useful but not standard. The lambda notation λ x.y is standard in a limited context but not generally useful. The concept of a mathematical expression is much older than the concept of a mathematical function". The minus sign is symmetric even if the minus operation is asymmetric. a−b is not the same as b−a, although a=b is the same as b=a. Touché. The minus sign is familiar to almost everyone, while the lambda calculus is familiar to almost noone. So it is easier to criticise the bad design of the lambda notation than to criticise the bad design of the minus sign. Bo Jacoby 06:36, 13 August 2006 (UTC)

And yet, somehow, mathematics muddles on.

A few thoughts:

On this talk page I gave examples of how functions may be presented in programming languages; however, I would like to confine the article itself to purely mathematical examples, given its name.
Having worked with symbolic computation systems and mathematical typesetting, I am very much aware that for computers we often use much more explicit notation.
Having read a great deal of mathematics covering a broad range of topics, I would also like to point out that most mathematics notation is written for humans, not computers; so we abuse notation, rely on context, and generally trust that a motivated and intelligent reader will find the terse notation friendlier than something totally explicit.

I offer as an exhibit of the difference the following MathML presentation markup for x−y.

<math xmlns="http://www.w3.org/1999/xhtml">

<mrow><mi>x</mi><mo>-</mo><mi>y</mi><mrow>

</math>

The standard itself makes clear that this is not meant for human consumption. We can also use content markup, which is no better. (I'll even omit the <math> tags.)

<apply><minus/><ci>x</ci><ci>y</ci></apply>

I'm surprised at your choice of targets. As it happens, lambda calculus and its notation is extremely useful; aside from its applications in the pure mathematics of recursive functions, it formed the basis for John McCarthy's creation of the heavily used and influential programming language LISP, and its functional programming language successors.

Your example of x^y is also puzzling. I cannot imagine this being used in an actual paper or text without enough context to clearly tell the reader the intent.

Besides, syntax always involves position. The matrix multiplication written AB is not the same as BA, and we can't blame the shape of the operator for any confusion.

This article, like mathematics in journals and textbooks, is written for humans. I believe our primary goal in discussing notation should be to tell readers about the most common notations they will see, and explain how to interpret them.

Often it helps to form a mental image of a typical reader. For an article like this, we can expect many readers will have little mathematics experience, else they would already be familiar with the term "function". In addition, we may attract a minority of readers with a great deal of mathematical experience hoping for some technical subtleties. If we write carefully, we can serve both groups. I commend to you the Wikipedia:Manual of Style (mathematics). --KSmrq^T 11:27, 13 August 2006 (UTC)

What is your solution to KYN's problem in Talk:Fourier_transform#notation once again? How do you express the shift operator ((c_k+1←k)←(c_k←k)) ? Will you suggest lambda calculus or lisp or MathML ? Rather than to argue that there is no problem you should try to solve the problem.

The suggestion (y←x) is primary for human consumption.

The problem of x^y is discussed in relation to defining 0⁰ . (0^y←y)(0)= 0 by continuity, while (x⁰←x)(0)= 1 by continuity. A lot of confusion and seemingly endless discussion arises as a result of lack of appropriate notation for these functions. (See for example Talk:Empty_product#00_is_indeterminate.3F).

I am not an expert on lambda calculus, and I do not understand the supposed advantage of writing λ x.y rather than (x→y) or (y←x).

Bo Jacoby 13:09, 13 August 2006 (UTC)

Please carefully read WP:NOR. Three different editors at Fourier transform told you Wikipedia is not the place to introduce your personal pet notation. You've been told that on this page as well. Believe it. --KSmrq^T 22:24, 13 August 2006 (UTC)

There is a difference between notation and research. There is no ban against notation. The ban is against what does any of the following:

It introduces a theory or method of solution; (y←x) does not.
It introduces original ideas; (y←x) does not.
It defines new terms; (y←x) does not.
It provides or presumes new definitions of pre-existing terms; (y←x) does not.
It introduces an argument, without citing a reputable source for that argument, that purports to refute or support another idea, theory, argument, or position; (y←x) does not.
It introduces an analysis or synthesis of established facts, ideas, opinions, or arguments in a way that builds a particular case favored by the editor, without attributing that analysis or synthesis to a reputable source; (y←x) does not.
It introduces or uses neologisms, without attributing the neologism to a reputable source. (y←x) does not.

If you know a more established notation expressing the same thing as (y←x), let's use it. I have no pet notation, but we need some notation. It's your problem as well as mine. Bo Jacoby 08:00, 14 August 2006 (UTC)

No, I have no problem with using common notation here; used wisely, it is clear. And I have searched both this talk page and the article history in vain for a citation of a reputable source to support the notation you propose. Since I have seen a wide assortment of mathematics yet have no recollection of having seen this notation, and since functions and their usual notation are very much standard mathematics, I can only conclude that your proposed new notation is original research. Feel free to use it as much as you like on talk pages, but expect it to be poorly received because no one will know what it means. It has no place in the article. --KSmrq^T 08:49, 14 August 2006 (UTC)

What is your 'common notation' for (sin(x)/x←x) ? Bo Jacoby 09:09, 14 August 2006 (UTC)

I also have lots of mathematics experience, including higher math, and have not seen that notation. Do you mean f(x)=sin(x)/x? Everything I've seen suggests that's the 'common notation'. The notation you proposed will not be understood by nearly as many people as the 'common notation'. -- Schapel 12:47, 14 August 2006 (UTC)

Hi Schapel. Yes, I mean that the equation, f(x)=sin(x)/x, for the function value, f(x), is true for all values of x for which the right hand side makes sense. Which function, f, solves that equation? What is the 'common notation' for f itself? f=??? Tell me. Bo Jacoby 08:32, 15 August 2006 (UTC)

I would say f=sin(x)/x. It usually isn't written that way so you can easily tell the constants from the variables, and also so that you can clearly tell a function of one variable from a function with two variables. For example, f(x)=sin(x)/x, g(x,y)=sin(x)/x, h(x)=k sin(x)/x. -- Schapel 12:59, 15 August 2006 (UTC)

The challenge is to have the function name alone on the left hand side of the equality sign, so that we get an expression for the function rather than for the function value. Do you suggest to strip away the parameters : f=sin(x)/x, g=sin(x)/x, h=k sin(x)/x ? Then you cannot tell the difference between f and g any longer. (My solution is f=(sin(x)/x←x), g=(sin(x)/x←(x,y)), h=(k·sin(x)/x←x).) Bo Jacoby 13:22, 15 August 2006 (UTC)

Sorry to jump in here, however as a student of mathmatics (university level), and therefore possibly a common type of reader, I would like to comment. I find issue with the words 'My solution' that you have used. From what I have read wikipedia is supposed to be representative of general use rather than personal preferance, therefore however useful and clear '[Your] Solution' is, Wikipedia is not the place to publish it. If you could publish in reputable source your solution to the issues you percieve to exist, and then experts accept the solution then maybe it will be worthy of addition. Sorry if I have spoken out of turn DougBrown 17:13, 2 November 2006 (UTC)

If you want the parameters in, why not put them where they belong in the first place? -- Schapel 18:15, 15 August 2006 (UTC)

In 'Talk:Fourier_transform#Where_to_put_it' some highly qualified people are seriously lacking an unambiguous notation. They have to settle with ambiguous expressions, sometimes complemented by explanations. It is unsatisfactory. Bo Jacoby 09:08, 16 August 2006 (UTC)

I just asked my wife, and she thought that f=(sin(x)/x←x) was some sort of limit. I asked her how to indicate that f is a function of x, and she wrote f(x)=sin(x)/x and said that's the only notation she knows to write that. She has a masters degree in statistics. -- Schapel 22:49, 15 August 2006 (UTC)

You did not ask her how to indicate sin(x)/x as a function of x rather than as a value, showing that x is a variable rather than a constant? My complements to your skilled wife. Let us take her word for it: there exists no standard mathematical notation for a function alone, only for a function value, which is sufficient in some cases, but not in every case. We must document this sad and embarrassing state of affairs in the article. Hopefully some expert mathematician will show up and teach us that we are wrong and provide an acceptable solution. Bo Jacoby 09:08, 16 August 2006 (UTC)

I still don't see the problem. In the functions above, you do not give the domain and range. Are they the reals? The complex numbers? Usually you don't give the domain and range of functions in a mathematical notation; they are implied from the context. You can also write y=sin(x)/x when it's implied that y is a function of x from the context. You can argue that these are "ambiguous expressions," but mathematics is a human language, not a computer language. All human languages have ambiguities. -- Schapel 12:10, 16 August 2006 (UTC)

[edit] Standard notation

This is a response to a comment signed Bo Jacoby 09:08, 16 August 2006 (UTC) above. There are two standard ways of indicating the function $f (x) = sin(x) / x$ that are explicit about the bound variable:

$x \mapsto \sin(x)/x$

λ x .sin(x) / x

The first is commonly used in category theory, the second in logic. In most written mathematics, it is enough to just say (in prose) what the bound variable is when confusion could arise. Since the entire community of mathematicians seems to have agreed that this is precise enough, new notation is apparently not necessary. The notation $\sin(x)/x \leftarrow x$ is completely nonstandard. A common way of defining a shift map (in the context of Fourier analysis, dynamical systems, etc.) is by saying that the shift $F (x)$ of a sequence $x = \langle x_n \mid n \in \mathbb{N}\rangle$ is the unique sequence such that $(F (x)) n = x n + 1$ for all n. This could also be written with different function notation: $(F (x))(n) = x (n + 1)$ . CMummert 12:00, 16 August 2006 (UTC)

[edit] Thank you very much

I suppose that this means that you can write

$f=(x \mapsto \sin(x)/x)=(\lambda x.\sin(x)/x)$ .

Can you omit the parentheses and write

$f=x \mapsto \sin(x)/x=\lambda x.\sin(x)/x$ ?

Can you write the shift like this

$F=x \mapsto n \mapsto x_{n+1}=\lambda x.\lambda n.x_{n+1}$ ,

or do you need parentheses:

$F=x \mapsto (n \mapsto x_{n+1})=\lambda x.(\lambda n.x_{n+1})$

or even

$F=(x \mapsto (n \mapsto x_{n+1}))=(\lambda x.(\lambda n.x_{n+1}))$ ?

Is the function expression used like the function name?

$(x \mapsto \sin(x)/x)(a)=(\lambda x.\sin(x)/x)(a)=\sin(a)/a$ .

Bo Jacoby 13:07, 16 August 2006 (UTC)

[edit] warning to editors

Other editors should be aware that the strong consensus policy against nonstandard personal notations has been explained to Bo many times, so you may be wasting your breath by arguing with him. On Root of unity, he tried to introduce the notation $1 1 / 2 = - 1$ . On Discrete Fourier transform, he tried to introduce his own "involutary" definition of the DFT. On Ordinal fraction, he tried to introduce a new fraction notation. Each time, he gave no warning to other editors, until challenged, that the notation was not (to his knowledge) used by anyone else. The same thing has apparently happened here. He gives all indication of continuing this behavior in perpetuity, which is sad since he clearly has mathematical training and is capable of making positive contributions. —Steven G. Johnson 16:17, 14 August 2006 (UTC)

Steven agrees that z^1/2 is a multivalued function of the complex variable z: "The expression z^1/2 can signify any of the solutions, x, to the equation x²=z". This is true for every complex number z. Steven also agrees that every real number is also a complex number. 'Complex' does not mean 'non-real'. So, 1 is a complex number. It is a standard mathematical procedure to specialize a formula by substituting a constant for a variable name. Substituting the complex number 1 for the variable name z in the above statement gives the specialized statement: "The expression 1^1/2 can signify any of the solutions, x, to the equation x²=1". Nevertheless Steven claims that it is 'nonstandard personal notation', and he has consistently been deleting my contributions. That I am not fighting edit wars or wasting my breath by arguing does not imply that Steven is right. Steven is simply not right in this matter. Bo Jacoby 08:32, 15 August 2006 (UTC)

It's absurd to pretend that Steven alone disagreed when we can read the talk pages and the histories; multitudes correct you every time they see you trying to introduce your own notation. Wasting your own time is up to you, but forcing other editors to waste their time correcting you for the same behavior repeatedly is the kind of thing that can lead to disciplinary action. --KSmrq^T 13:16, 15 August 2006 (UTC)

Which part of the above mathematics is nonstandard ? I do not pretend that Steven is alone, but he is the prime mover. Bo Jacoby 13:35, 15 August 2006 (UTC)

No, Steven is not alone. Wikipedia is not the place to introduce non-standard notation. There is no doubt that current matematical notation could be improved, but Wikipedia is not the place to do it. (My own personal bugbear is =o (little o notation), the only place in all mathematics where an equal sign is part of a notation that is not an equivalence relation.) But the place to introduce your ideas about mathematical notation is in an article for a refereed journal, not an encyclopedia. I suggest you try The American Mathematical Monthly but they are pretty conservative -- they wouldn't publish my article on little o. On the other hand, they did publish my article on why 2 + 2 = 2 * 2, so you never can tell. It's worth a try. But not in Wikipedia. Rick Norwood 14:05, 15 August 2006 (UTC)

Hi Rick. I am interested in your suggestion for an alternative to the little oh notation. Why not put an abstract on your user page? See also Degree_of_a_polynomial#The_degree_computed_from_the_function_values. And now please answer my question: "Which part of the above mathematics is nonstandard ?" Neither z^1/2 nor 1^1/2 is nonstandard notation. See Exponentiation#Complex_powers_of_complex_numbers. It does not become nonstandard just because Steven or you do not use it. We agree that Wikipedia is not the place to introduce non-standard notation. Do we agree that Wikipedia is the place to use standard notation? Bo Jacoby 11:57, 16 August 2006 (UTC)

The notation 1^1/2 is not nonstandard. It indicates the square roots of 1, which are -1 and 1. The problem is that you used that standard notation in a different way from the standard meaning. Wikipedia is not the place to introduce nonstandard meanings to standard notations. -- Schapel 12:52, 16 August 2006 (UTC)

Did I ? Bo Jacoby 13:23, 16 August 2006 (UTC)

Yes, you used the notation 1^1/2 to mean -1 and not 1. That is not the standard meaning of that notation, and actually creates ambiguity where none existed before. -- Schapel 13:40, 16 August 2006 (UTC)

Who wrote 1^1/2 meaning +1 ? Bo Jacoby 14:08, 16 August 2006 (UTC)

I'm not sure what you're asking. 1^1/2 means one raised to the one-half power in standard notation. The values +1 and -1 are one raised to the one-half power. -- Schapel 15:23, 16 August 2006 (UTC)

I don't put my views on "little o" in Wikipedia because this is not the place for them. Nor is this the place to push for wider use of the multivalued 1/2 power. If you type 1 ^ (1/2) into a calculator, it returns 1, not {1, -1} In other words, the default for fractional powers is the principal value (if defined), not the set of values. When the author of a mathematics article wants to use fractional powers in some non-standard way -- as, for example, a function defined between two Riemann surfaces -- they mention that in the introduction to the article or book. Rick Norwood 14:23, 16 August 2006 (UTC)

Does Rick's calculator support complex numbers? If not, the argument is invalid. If you want to express the number 1, then write the symbol 1 rather than the expression 1^1/2. Actually 1^1/N is the systematic notation of an N^th root of unity, but it must be mentioned that it is ment to be a primitive N^th root of unity such as e^{2·π·i / N}. Notations like ω_N are ad hoc and should be banned. Some editors oppose the notation 1^1/N and so I do not pursue it, but that does not imply that their arguments are valid. Some people don't think that they can learn from me, and they are right. :-) Bo Jacoby 09:12, 17 August 2006 (UTC)

Bo writes, "1^1/N is the systematic notation of an N^th root of unity". Notice that he doesn't claim that it is a standard notation (although he carefully avoids saying that it isn't), and he has not been able to cite a single reference that uses this notation. This has all been covered on Talk:Root of unity where Bo tried to introduce the same notation, and is somewhat offtopic here. Conversations with him go round and round, because he keeps trying to convince you that his notation is better, and you keep explaining that this is irrelevant — Wikipedia policy is to use established mathematical notations, for better or for worse. —Steven G. Johnson 17:40, 17 August 2006 (UTC)

Conversation with Steven go round and round, because he keeps trying to convince you that his notation is standard, and you keep asking him for documentation for that claim. Seaching through the list of ISO standards showed me no standard for mathematical notation. But there is consensus that aⁿ is standard notation for the n^th power of a, and that is all I need. I suggest that this ad hominem discussion be moved elsewhere. It does not belong on this talk page. Bo Jacoby 11:30, 18 August 2006 (UTC)

Mathematical notation is a de facto standard, not a de jure standard. If you use nonstandard spelling of words or use a word to mean something different from its standard meaning, your edits will be reverted. Similarly, if you use nonstandard notation or notation to mean something different from its standard meaning, your edits will also be reverted. There's nothing ad hominem about that. -- Schapel 12:45, 18 August 2006 (UTC)

That blows Stevens argument. The established de facto standard is that z^1/2 is a multivalued function of the complex variable z. Bo Jacoby 17:11, 20 August 2006 (UTC)

It damages Steven's argument. It destroys yours, as 1^1/n, if given a single value, is always 1. That's why it's called the principal value. — Arthur Rubin | (talk) 17:59, 20 August 2006 (UTC)

Um, of course it's multivalued; no one disagrees with this. But, as I and others have explained repeatedly, when it is written in a context where a single number is implied (i.e., most contexts, unless otherwise indicated explicitly), then the default meaning is the principal value. The most familiar example, of course, is the square root, which is by convention taken to be positive, and this is where your

1 1 / 2 = - 1

suggestion will be most jarring to readers, but the same is true for other powers. Your implication above that

1 1 / 2

should have a different meaning when talking about complex and real numbers is equally jarring. (That's why essentially every programming languange that supports complex numbers, from Maple to Mathematica to Matlab to the ISO C99 standard, defines the complex exponentiation operation/function to be the principal value in numeric calculation.) —Steven G. Johnson 18:18, 20 August 2006 (UTC)

Anyway, this is going offtopic; if Bo wants to try to argue for $1 1 / 2 = - 1$ (again), the proper place is Talk:Root of unity. Or, more appropriately, since multiple editors have told him this is counter to policy, he can make an open policy-change proposal on the appropriate policy Talk pages (good luck). If he feels I have pursued an unjust vendetta against him, he is free to file a complaint on Wikipedia:Requests for arbitration, although I suspect that such a complaint would backfire. —Steven G. Johnson 18:18, 20 August 2006 (UTC)

While the principal N^th root of unity is always 1, a primitive N^th root of unity is never 1, except for N=1 . Steven emphasizes that "the standard choice is the principal value unless otherwise indicated explicitly". I agree, and I did indicate otherwise explicitely. Stevens position is that even if any N^th root of z may be called z^1/N for any complex value of z except 1, a primitive N^th root of unity must not be called 1^1/N, but must be called e^{2·π·i / N}, a transcendental expression, disguising the fact that this is an algebraic number.

This has been offtopic from the very beginning when Steven entered this "warning to editors". Even if Stevens argumentation is invalid, I did stop using the notation 1^1/N. Steven construed this as a victory, which it is, and as an indication that he is right, which it is not. We have had discussions, and his different arguments are refuted one by one, but that does not alter his opinion. The only one who is talking about "an unjust vendetta" is Steven's bad concience. I called it 'ad hominem', which it is, as it is a warning against a person in general, rather than against a specific opinion. Bo Jacoby 20:55, 20 August 2006 (UTC)

You're misstating my position and our discussions, but I'm not going to (re-)argue it here (for the nth time) as this is the wrong forum. Thanks for stopping your battle to introduce the

1 1 / n

notation, by the way, although you stopped only after re-inserting it into three separate articles at three separate times. I have no guilty conscience; "vendetta" was simply a paraphrase of your description of me as "prime mover" in "consistently deleting" your "contributions". As for my warning being "offtopic"—pointing out to other editors that they were dealing with someone who has repeatedly ignored policy in the very matter under discussion (introducing nonstandard personal notations) was, sadly, perfectly relevant. —Steven G. Johnson 21:00, 20 August 2006 (UTC)

I am not ignoring policy. The notation 1^1/N is not against policy. It is neither new research nor nonstandard notation. All Steven's arguments in that direction have by now been refuted. The only problem with that notation is that it was new to Steven, and Steven considers himself to be an authority on the subject. Steven's attitude is against the spirit of wikipedia. He should let conclusions depend on arguments and not the other way round. Some editors were not aware of the connection between fourier transform and roots of unity, and my contributions, rather than Steven's, arose that awareness. By the way, what is the point of putting the word contribution in quotes? Bo Jacoby 06:19, 21 August 2006 (UTC)

As I've already explained before, the problem with the notation you were using is not that it is nonstandard, but you gave the notation a nonstandard meaning. That's why your edits were reverted. If you use the word chair to mean table, of course someone else will correct the mistake. Arguing that chair is a standard English word and therefore is correct is stupid and pointless if you used it to mean something besides chair. -- Schapel 12:21, 21 August 2006 (UTC)

1^1/N is multivalued like 'furniture', not singlevalued like 'chair'. 'Furniture' can mean 'table', it doen't necessarily mean 'chair'. Likewise 1^1/N can mean e^2·π·i/N , it doesn't necessarily mean 1. Bo Jacoby 13:33, 4 September 2006 (UTC)

Exactly. That's why this notation was called non-standard. -- Schapel 17:45, 7 September 2006 (UTC)

AAGGGHHHH! I have just spent a long time reading his discussions and if I had read this I would not have done, and possibly spent time reading more of the excellent infomation on mathmatics Wikipedia has to offer. Has he defined 1/0 yet? DougBrown 17:20, 2 November 2006 (UTC)

[edit] first function definition

I'm not a mathemetician. The first definition of a function states that 'the binary relation denoted by "less than" contains the ordered pair (2,5) because 2 is less than 5'. In that case, doesn't the same relation also contain the ordered pair (2,6)? Doesn't this then contradict the next paragraph, which says that if (a,b) and (a,c) are in the set, then b must equal c?

The less than relation is not a function. The article could be more clear about this. CMummert 00:23, 23 August 2006 (UTC)

I suspect many computer programmer would hold that a binary relation is a function: one that maps ordered pairs to the set {true, false} :-) - Fredrik Johansson 16:03, 23 August 2006 (UTC)

I think that's a red herring in this context. The definition in the article is completely standard. I agree it is common in computability theory to identify sets with their characteristic functions, but this identification is not common in most areas of mathematics. Since the article title is Function (mathematics), it's fine. I take it from the :-) that you're not serious anyway. CMummert 16:17, 23 August 2006 (UTC)

I suppose we could really picky here. From the text: "A function is a binary relation, f, with the property that for an element x there is no more than one element y such that x is related to y." Seems to me this implies there can be no y, "... no more than one ..." when in reality there is "one and only one" y for a given x. The text is inconsistent in it's definition of a function.

[edit] a vital article should be good for reference

I think we all agree on that this is a vital article.

It should be as clear as possible, but it is not clear at all, since several (not only 2) contradictory definitions are given, and sections are unrelated.

I think it's OK to start by giving some examples, so everybody has a rough idea of what we're talking about. (Even if I am about 99% sure that no person reading this article does not already roughly know what it is about.) But I'm sure that (much) over 50% of those who come to this page want some precision about functions. They will be heavily disappointed, since they find only contradictions.

When starting the definition, I think it is not too complicated to distinguish right from the start the range from the codomain, and the function from its graph (which is defined later on, but equal to the first "definition", without this being mentioned).

In other words, a function is defined by 3 things : the "departure set", the "arrival set", and its graph. This insofar more than concensus seems to have been found on defining a function as a binary relation. Once this concensus is reached, one must accept the consequences.

No one would impose on a binary relation to be always total (else a strict order like "<" would not be a binary relation on the interval [0,1] since for x=1 there is no y such that x < y ; and the same would be the case for most partial orders). This implies that not every function is a map (for which the "domain of definition" is equal to the "departure set"), but I will not insist on this here. (I can live with the fact that on wikipedia, in contrast to all classical mathematical books as well as all current work in mathematics, this is (explicitely) called a partial function [while this term has another meaning for mathematicians].)

Also, knowing "from where" the function comes and "to where" it goes is important not only for being able to question whether it is surjective (as alluded to), but also for function composition.

Also, this article should not be about functions in computer science. The notion of "computable function" should not be discussed on this page, since in this context, "function" is not used in the sense of the Definition given here (binary relation). (The set of functions, or maps, from N to N, aka N^N, is not countable, although any given function f=(f(x))_x∈N from this set is easily computable since it is sufficient to look up the value in the "table" (f(x))_x∈N.)

I think this needs heavy cleanup (a starting point could be to delete here what is duplicate with the page "partial function"). — MFH:Talk 14:10, 13 October 2006 (UTC)

I agree that this article could be substantially cleaned up. I second the opinion that partial function should be merged here, and this article significantly editied.

One difficulty with your proposal will be reaching a consensus about what the formal definition of a function is. I would say that the definition of a function is literally the same as its graph. So when you say the function and its graph must be distinguished, I can only guess what you mean. I also don't know what you mean when you say that there is another meaning for partial function in mathematics; a partial function on a set A is just a function whose domain is a proper subset of A. CMummert 15:47, 13 October 2006 (UTC)

I agree the "two definitions" in the article are not ideal. I wrote a possible replacement defintion here. Feel free to edit it or comment on it here. CMummert 16:37, 13 October 2006 (UTC)

On a regular basis we see editors wanting to make mathematics consistent. It is impossible not to sympathize; it is also impossible to do. Wikipedia documents, it does not dictate nor editorialize.

Some authors treat a function as a set of ordered pairs satisfying certain constraints; this would be a "graph" approach. Other authors insist that the domain and codomain (not the range) are essential to include as well. We also see "function" used in contexts like "Dirac delta function", or like "rational function" in algebraic geometry (undefined at the zeros of the denominator polynomial), and so on. And we see alternative names like "map", which sometimes are synonymous and sometimes not.

What to do? We first emulate the maps of transit systems, like the London Tube map, and focus attention on the information needed to guide the reader through the maze of possibilities. However, we also later include enough details to be reasonably complete. This demands writing skills and a breadth of knowledge few individuals possess, but perhaps the Wikipedia "hive mind" can overcome that limitation. --KSmrq^T 02:18, 14 October 2006 (UTC)

Resp. to KSmrq: that is a very accurate opinion about the general state of things. So what is your particular opinion about this particular situation? Did you look at my proposed change, and did you have any opinion about it? I agree (somewhat) with MFH that the current article could be improved. CMummert 02:28, 14 October 2006 (UTC)

[edit] Merge?

(Moved from Talk:Partial function)

I say this should not be merged into function (mathematics). 'Partial function' is a useful clear idea, much used in computer science. Charles Matthews 11:03, 14 October 2006 (UTC)

Do those reasons really support having a seperate page? The material on partial functions would not be lost if it were merged and redirected here. Unlike domain and codomain, few people are going to run into the phrase partial function until well into college, so the article on partial functions is of less general interest than other vocabulary articles on functions.

Another issue. There is a great deal of repetition among many the vocabulary articles such as: Domain (mathematics), Range (mathematics), Codomain, Injective function, etc. And these articles individually have their own problems, including references and POV claims (which I am noting here so I will remember it later). I think these vocabulary articles could use a link to this article, which could be put at the very top as an italicized header:

For a general discussion of functions, please see the article Function (mathematics).

CMummert 12:57, 14 October 2006 (UTC)

Partial function is a self-contained article and better be kept separately. This function (mathematics) is already huge, adding even more to it would make it hard for people to look up relevant information that way.

I also disagree with putting the note

For a general discussion of functions, please see the article Function (mathematics).

The term "function" is proeminently linked within the text in the appropriate places in those articles, and that should be enough. Oleg Alexandrov (talk) 17:51, 14 October 2006 (UTC)

[edit] Definition of a function

I will try to find a referance to this fact but I was told recently by a tutor of mathmatics that the strict definition of a function is that for each function there must be a corresponding inverse function and therefore each input can only correspond to one output AND each output can only correspond to one input.

So f(x)=x^2 can only be a function if restricted to x >= 0

I know that this is not the place to discuss mathmatics, however if the above is true I think it should be noted. DougBrown 16:32, 2 November 2006 (UTC)

That's an Injective function, f(x)=x^2 is a function without the restriction x >= 0. skip (t / c) 14:00, 2 November 2006 (UTC)

Thanks, Do you think that a short mention of Injective Functions would be useful on this page? with a link to the Injective function page? DougBrown 16:32, 2 November 2006 (UTC)

It's already mentioned in the vocabulary section, linked from "injections". It may be helpful to link to the Horizontal line test and/or Vertical line test articles. skip (t / c) 07:19, 3 November 2006 (UTC)

[edit] Bijection, injection and surjection redirects here

I see that Bijection, injection and surjection has been redirected here; however, there was a lot more detail on the last version of that page before it was changed to a redirect^[1] that doesn't seem to be here anymore. I read through the talk page and don't see any discussion of this merge; am I missing something? It seems to me that either that page should be restored, or its redirect is pointing to the wrong place, or there's a bunch of info that needs to be added here. Personally, I lean towards reverting the redirect on Bijection, injection and surjection - any thoughts? Perel 04:37, 5 December 2006 (UTC)

Revert. This merge/redirect was a dubious idea that should have been discussed first. --KSmrq^T 06:19, 5 December 2006 (UTC)

I agree. Revert. Rick Norwood 13:29, 5 December 2006 (UTC)

Done. If anyone strongly opposes this revert, start an AFD and mention it here. Perel 03:21, 6 December 2006 (UTC)

The more difficult question is why there is a page Bijection, injection and surjection as well as pages bijection, injection, and surjection. I can't see any benefit to this redundancy. CMummert 13:05, 6 December 2006 (UTC)

The closer we look, the stranger the situation appears. First, we have the obvious redundancy of content. Second, "Injection" itself is a disambiguation page (which makes sense). Third, we have "Bijection", but "Injective function" and "Surjective function". Fourth, although there is redundant content, there is also distinct content. Fifth, we have nearly duplicate figures, but in the joint article they are PNGs, while in the individual article they are SVGs.

So the question would seem to be, is there enough broad content for the joint article to so limit itself (rather like the manifold article)? If not, should it absorb the others or should it disappear? One concern: I would find it annoying if a passing reference to a surjection dumped me at the beginning of an omnibus article, when all I wanted was a quick definition; I do not believe we can rely on editors to link to a subheading, nor to avoiding renaming subheadings.

Meanwhile, the mathematical formatting in all could use some polish. --KSmrq^T 15:11, 6 December 2006 (UTC)

I would keep the three separate pages, since most editors will link to the words surjection, bijection, etc. when writing other articles. Over 200 articles link to bijection, only a handful link to the joint article. Fixing all the double redirects if bijection were removed would be a pain, and even then more would appear.

Since the function article already has links to the individual articles, I don't see why the joint article needs to be preserved. The content in it can be divided between the individual articles and the main function article. I suppose the joint article could be made a redirect to the main function article, to avoid an AFD. CMummert 15:35, 6 December 2006 (UTC)

Yes, this does get stranger and stranger. I'm thinking that we may want to rename "Bijection" "Bijective function" for consistency, and remove the joint article. That really shouldn't be done until there's consensus that anything relevant from the joint article has been moved back into the individual articles. Perel 16:11, 6 December 2006 (UTC)

Given the options contemplated, we should move this discussion to the joint talk page. But first, if there is very little unique material in the joint page, perhaps that extract could move to the function page. Thoughts? --KSmrq^T 05:55, 7 December 2006 (UTC)

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