Talk:Sign function

From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Mathematics (Rated Start-class)
WikiProject Mathematics
This article is within the scope of WikiProject Mathematics, a collaborative effort to improve the coverage of Mathematics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Mathematics rating:
Start Class
Priority unassessed  Field: Unassessed
The assessment information for this article is not complete. Anyone can add information about the quality, significance, or subject area of an article.

removal announcement[edit]

I removed the following

Also, if the step function h0(x) is thought of as a mathematical switch, with h0(x) = 0, then the signum function can be expressed as
 \sgn x = h_0 (|x|) (-1)^{h_0(-x)}.

as I do not understand what it adds.--Henrygb 14:34, 29 Mar 2005 (UTC)

Since h(x) can be different things, it looks like someone was just leaving a note of how to calculate sgn for people who are used to using another definition of h(x)? Who added it in the first place? - Omegatron 16:12, Mar 29, 2005 (UTC)

isn't it kinda dumb to have this and the signum function page, which are talking about the same thing????Scythe33

Link to "sign" page[edit]

In the first paragraph, the word "sign" in "extracts the sign of a real number" was linked to the Negative and non-negative numbers page. There doesn't seem to be a page for the correct concept. I have removed the link. Iggle 08:27, 27 March 2006 (UTC)

Pronunciation?[edit]

Does anyone know the correct pronunciation of "signum"? I'd intuitively say "sigg-numb", but in context, "sigh-numb" might also be correct. Might be worth putting in.


Hmmmmm... I checked Wiktionary, but it didn't provide a pronunciation. And dictionary.com provided no entries. I usually pronounce it "sigg-numb," like you. -- He Who Is[ Talk ] 18:17, 22 June 2006 (UTC)

Since it is (presumably) Latin, the "g" should be pronounced. Zaslav (talk) 01:09, 27 February 2008 (UTC)
"Signum" pronounces as "SIN-yoom". How to Pronounce Latin --Octra Bond (talk) 05:48, 8 June 2008 (UTC)
The pronunciation given at that site is the pronunciation of Ecclesiastical Latin, as is made quite clear there. If the word signum is pronounced as part of Latin liturgy, for instance when reading in the liturgy of Christmas from the Gospel of Luke 2:12 Et hoc vobis signum : invenietis infantem pannis involutum, et positum in præsepio, you can expect to hear "SEEN-yoom" (or, if the speaker is Italian, "SEEN-yoom-uh"). In Classical Latin, as far as we know, the 'g' was hard, as in "SEEGG-noom". In addressing an English-speaking audience, I'd go with "SIGG-numb", as if the word has become an accepted English word, in analogy to how the 'g' is pronounced in 'magnificent' and 'recognition'.  --Lambiam 19:22, 8 June 2008 (UTC)

Complex Signum[edit]

I've added a section on the signum of complex numbers. Feel free to critique or remove, if you feel it is extrenuous. -- He Who Is[ Talk ] 18:14, 22 June 2006 (UTC)

  • I've just added a description of csgn from Maple, which is another generalization of signum to complex numbers. I think it was neccessary because csgn redirects here but there is no mention of csgn. Rjgodoy 02:44, 30 May 2007 (UTC)

Signature Function (permutations)?[edit]

Sgn redirects here, but there's no mention of the signature of a permutation matrix which is also denoted with the sgn() function. That's actually what I was trying to find, and only happened to stumble across it linked from another article. 142.59.195.50 19:36, 31 March 2007 (UTC)

  • Hello, and thanks for the comment. I have added a note about sgn(σ) that suggest Levi-Civita symbol. The generalization to n dimensions of Levi-Civita pseudotensor is another form for the sgn(σ) function you refer. Rjgodoy 02:29, 30 May 2007 (UTC)

Generalized signum function[edit]

The first paragraph of this section says "~\varepsilon(x)^2 =1~ everywhere, including at the point ~x=0~" but the second says "~\varepsilon(x)~ cannot be evaluated at ~x=0~". Why does it conflict itself? --Octra Bond (talk) 05:55, 8 June 2008 (UTC)

Generalized functions do not behave like ordinary functions. The paragraph pertains to a construction in the algebra of generalized functions developed by Yu. M. Shirokov; see Wikisource:Algebra of generalized functions (Shirokov). In that algebra, δ(x)2 = 0 at x = 0 even though δ(0) = +infinity. The notation is (in my opinion) misleading, since what is being multiplied are the generalized functions themselves, and a notationally clearer statement would have been that ε)(x) = 1 and δ)(x) = 0 for all x, in which denotes the (non-commutative) operation of multiplication for generalized functions.  --Lambiam 18:40, 8 June 2008 (UTC)

Undefined?[edit]

The textbook "Calculus- a complete course" by Robert A. Adams says that sgn(x) is equal to 1 if x > 0; equal to -1 if x < 0 and that it is undefined when x is equal to 0; that is to say that:

 \sgn x = \begin{cases}
-1 & \text{if } x < 0, \\
undefined & \text{if } x = 0, \\
1 & \text{if } x > 0. \end{cases}

The explanation is verbatim "The value of the sgn(x) tells whether x is positive or negative. Since 0 is neither positive nor negative, sgn(0) is not defined." This makes sense seeing as how the sign function is x/(abs(x)), and when x = 0 one would get 0/0. This directly contradicts this article, which both has a dot at (0,0) and states that:

 \sgn x = \begin{cases}
-1 & \text{if } x < 0, \\
0 & \text{if } x = 0, \\
1 & \text{if } x > 0. \end{cases}

Someone mentioned above that since the signum function is a generalized function, sgn(0)=0 can apply but that doesn't mean that it is true. If somebody knows anything about this, I'd like this to be disambiguated and (if wrong) fixed. A small explanation about 'why' it isn't undefined would also be nice if somebody understands that. --BiT (talk) 14:25, 10 November 2008 (UTC)

This, this, this and this page from Matlab are sources that sgn(0) is defined, but isn't a published textbook more reliable than some articles on the internet? --BiT (talk) 14:32, 10 November 2008 (UTC)

Leaving it undefined at 0 makes the derivative make more sense (as it too would be undefined at x=0). —Preceding unsigned comment added by 142.162.24.209 (talk) 04:59, 5 February 2009 (UTC)

Leaving it undefined at 0 makes no difference for the derivative since limits are defined to require an approach from both sides. Defining the sgn to be 0 at 0 allows for a more fluid definition of the matrix determinant in terms of the Leibniz formula. Doing so allows the sum to be taken over all arrangements up to N, and not just all permutations. It makes more sense from a combinatorial point of view. Antares5245 (talk) 00:20, 14 December 2009 (UTC)

Ancient post, but here goes, first the complicated answer: The two functions are equivalent under the L2 norm since they differ in only one point. This implies for example that the fourier series for both is the same. One possible reason to favor the sgn(0)=0 convention is that the fourier series for sgn(x) converges (pointwise) to 0 in x=0. Making a distinction though between the two functions is mostly irrelevant. Almost everyone uses the zero convention or states explicitly what the behaviour at zero is.--217.84.27.165 (talk) 11:13, 13 November 2011 (UTC)

math on Wikipedia[edit]

Is it just me or is math on Wikipedia either exceedingly complicated it's not worth trying to understand, or such within the realm of basic common sense it is laughable? "Any real number can be expressed as the product of its absolute value and its sign function"

173.183.79.81 (talk) 23:45, 15 March 2011 (UTC)

It's not just you and not just Wikipedia; all mathematics falls into one of these two categories. --catslash (talk) 22:12, 18 March 2011 (UTC)
173, sign in and help make it better. Cliff (talk) 05:20, 1 June 2011 (UTC)
It may be common sense, but it is well worth stating basic mathematical axioms in an encyclopedia. You may as well say "I could have thought of the whole concept of signum, and therefore it doesn't need an article." Indeed, signum is very very simple, and anyone with common sense could have come up with its definition, drawn the simple graph on the article, worked out the axioms and its relationship to the absolute value function, etc. But it is still useful to have such simple definitions available, so we can derive more complex things from then. The axiom you quote is used in this article to show a few other interesting properties. —MattGiuca (talk) 07:04, 25 August 2011 (UTC)

Algebraic representation[edit]

As far as I can tell, the entire point of the section on algebraic representation is "couldn't we express the signum function without using a conditional?" In other words, the basic definition of signum has a three-way switch using the condition brace; can we do it using normal (non-conditional) mathematics?

It seems like it could be simplified as \frac{x}{|x|} or \frac{x}{\sqrt{x^2}}, but I suppose that would be undefined when x is 0. (This would lend credence to the above suggestion that signum be undefined when x is 0.)

Failing that, I don't really see the point of this section. It doesn't cite any sources to show where this complicated formula came from. The fact that it requires n be a certain number of decimal digits is mathematically messy and requires more special cases than the simple definition of signum in the first place. What is the point / use of this formula? —MattGiuca (talk) 07:17, 25 August 2011 (UTC)

I thought just the same. Your comment got no response in 3 months, so I deleted the section. Stephanwehner (talk) 07:01, 3 December 2011 (UTC)

Citation needed?[edit]

Really? Is a citation really needed for the derivative? Surely this is a basic calculation. It follows trivially from the line below it about the Heaviside step function(which doesn't say citation needed).--217.84.27.165 (talk) 11:21, 13 November 2011 (UTC)

The same thought occurred to me - that this is essentially the definition of the delta function. However, it could be that mathematicians have scruples about differentiating a discontinuous function, which I lack. Perhaps KlappCK [1] could explain the problem? --catslash (talk) 00:05, 14 November 2011 (UTC)
The purpose of a citation here is to increase the reliability of the article and to solidify it's relation to other functions. I have had to derive the result myself, starting with the Heaviside step function, so I am comfortable with the definition, but this page isn't just for me, or either of you, for that matter. Furthermore, the derivative is not universally defined (if you have ever used Mathematica, D[Sign[x],x] returns Sign'[x] because it does not "know" the derivative of the sign function). However, one could point to the definition for the sign function in terms of the Heaviside step function [2] and then to the definition of the Heaviside step function's derivative [3] in a footnote showing how these two pieces of information and simple differentiation yield the result with a link to said footnote next to definition in the body of the article. This would seem satisfactory to me, anyway. KlappCK (talk) 20:09, 27 December 2011 (UTC)
As an aside here, I believe that it may be beneficial for overall readability to move the definition of the sign function in terms of the step function up about the definition of the derivative of the sign function and use the former definition to derive the latter.KlappCK (talk) 20:29, 27 December 2011 (UTC)
I made the changes I had proposed. See what you think.KlappCK (talk) 21:25, 27 December 2011 (UTC)

Article title[edit]

Isn't "signum function" the original and still most common name for this function? I think the lead section of the article should be changed to something like: "In mathematics, the signum function (from signum, Latin for "sign") is an odd mathematical function that extracts the sign of a real number. This function is also sometimes called the sign function, although this may lead to confusion with the sine function. In mathematical expressions, the signum function is often represented as sgn."

Also, I think the article title should be "Signum function", not "Sign function". Isheden (talk) 16:59, 7 March 2012 (UTC)