Talk:J-invariant

Notation bugs

In the beginning of the article j(i)=1728, and in the end j(i)=1. One has something to do with it. — Preceding unsigned comment added by 95.84.228.116 (talk) 10:41, 22 June 2013 (UTC)[reply]

There is a notational problem in this and related articles; standard notation seems to be that $J=g_{2}^{3}/\Delta$ and j=1728 J. I'll try to review correctness/fix this when I have the chance. (where $g_{2}=60\sum (m\tau +n)^{-4}$ linas 03:54, 20 Jun 2005 (UTC)

I have introduced the alternative notation J=j/1728 at the end of the article to fix this bug in the (somewhat excessive) list of special values. N.Nahber (talk) 21:39, 11 September 2016 (UTC)[reply]

If It can be of any help I think I remember there are two notations for Eisenstein series too: $g_{2}$ or $G_{2}$ , correspoing to different conventions (leading coefficient one, integer coefficients, norm 1 etc...)

Why is the elliptic modular function attributed to Klein? The fundamental domain of the modular group was known to Gauss. It was studied by Kummer, and by Hermite (1858) in connection with the solution of 5-ics, and defined by Dedekind in a remarkable paper in Crelle ?1878. Klein became involved after Dedekind. He defined j in terms of absolute invariants of Int dx/y, y^2 = 4-ic in x. After a fw pages he reverts to Dedekind's definition. [John McKay] May be Atkin should be consulted?]

Confusing article

"the Fourier coefficients for the positive exponents of q are the dimensions of the grade-n part" - It's not clear what n is. Does it mean the coefficient corresponding to q^n in the q-expansion?

"the rate of growth of ln(cn) is" - cn is never defined in the article. Does it mean the coefficient corresponding to q^n? Also, the asymptotic formula shows the growth of ln(cn), not its rate of growth.

"there are exactly 6486 of them [functions]... (see here for the complete list)" - The link is to a document which appears to not define a single function. For example, "14A0 14B 28B 1 1 1 2C0 71B0 14" does not seem to be a function. —Preceding unsigned comment added by 209.67.107.10 (talk • contribs) 17:54, 6 July 2009

The three quotations were added respectively by User:Nbarth (20 October 2007), by User:Gene Ward Smith (24 May 2006), and by User:R.e.b. (14 May 2006). I have drawn the attention of these three editors to the above comment. However, Gene Ward Smith has not edited since 24 December 2008, so a response is perhaps unlikely. JamesBWatson (talk) 11:48, 10 December 2009 (UTC)[reply]

Hi James – thanks for drawing my attention to this.

Anon (209.67.107.10) is correct, the grade-n dimension corresponds to the coefficient of

q^{n}.

I’ve clarified this, and included a link to the Griess algebra as the first example.

—Nils von Barth (nbarth) (talk) 19:37, 10 December 2009 (UTC)[reply]

I have removed the doubtful material in the other two cases: if anyone can restore and clarify them please do so. JamesBWatson (talk) 10:33, 14 December 2009 (UTC)[reply]

Elliptic modulus

Elliptic modulus redirects here but is not mentioned.--JohannesBuchner (talk) 14:04, 4 November 2014 (UTC)[reply]

I have edited the section J-invariant#Alternate_Expressions and clarified that the modular lambda function is the square of the elliptic modulus. Titus III (talk) 00:13, 6 November 2014 (UTC)[reply]

Nonsensical statement

The section Special values begins as follows:

"The $j$ -invariant vanishes at the "corner" of the fundamental domain at

\;\;{\tfrac {1}{2}}\left(1+i{\sqrt {3}}\right).

Here are a few more special values (only the first four of which are well known; in what follows, $j$ means $J /1728$ throughout):

BUT: There is no capital-J function defined anywhere in the article.

I see that someone already began a section about notation problems, but that was 2.5 years ago and this problem still persists.

Would someone knowledgeable in this subject please fix this? E.g., by defining a new function that is j(τ)/1728 instead of using the same notation both for the standard j and a nonstandard j ???

Or else please use the simpler idea of just stating that the list of Special values is a listing of j(τ) /1728 ???Daqu (talk) 19:40, 19 January 2016 (UTC)[reply]

I have cleaned up this inconsistency by introducing the notation J=j/1728 used by Mathematica N.Nahber (talk) 21:25, 11 September 2016 (UTC)[reply]

Mathematica notation may be consistent with mathematics, but it may not be.

The correct area to check for proper notation is mathematics, not Mathematica.

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Why does one section suddenly change the notation?

The section Special values suddenly seems to use capital J instead of what the rest of the article uses: a lowercase J ("j").

Without any explanation. If the article defines the capital J function somewhere, I haven't found it (and it would not be easy to find).

Can someone knowledgeable on this subject please make this much clearer?50.203.182.230 (talk) 17:37, 31 October 2019 (UTC)[reply]

As explained towards the beginning of that section,

J(\tau )={\frac {1}{1728}}\cdot j(\tau )

. So these are two distinct functions, albeit differing only by a constant (multiplicative) factor. Turgidson (talk) 19:13, 31 October 2019 (UTC)[reply]

Then there is never any reason to use both of then at different places in this article.

Just state the relationship, and then use (lower-case) j for the rest of the article.

Otherwise this becomes terminally confusing.

Unexplained terminology

The section Class field theory and the j-invariant contains this sentence:

"If τ is any CM point, that is, any element of an imaginary quadratic field with positive imaginary part (so that j is defined), then j(τ) is an algebraic integer."

But the meaning of "CM" has not been explained anywhere in the article before this usage.

I hope someone knowledgeable about the subject can fix this bad writing. 2601:200:C000:1A0:3998:89DD:A33:8E64 (talk) 03:31, 25 September 2021 (UTC)[reply]

CM stands for "complex multiplication" here. This is suggested (not explicitly) later in the section so I added a wikilink there if that can help. TheMathCat 05:45, 25 September 2021 (UTC)[reply]

A cube of what?

The first sentence of the section Definition contains the phrase

"with the third definition implying j(𝝉) can be expressed as a cube".

It may be possible for some readers to guess what kind of cube is intended by this statement.

But since this is an encyclopedia article, it would be much better to state explicitly what kind of "cube" is referred to here.

Is it "a cube in the field of meromorphic functions on the upper half plane" ?

Of course, the answer is Yes, but I mean: Is this the kind of "cube" referred to in the phrase I quoted? 2601:200:C000:1A0:F4CF:60FC:DCAC:F9C5 (talk) 21:43, 5 June 2022 (UTC)[reply]

Unclear statement

The section Definition begins as follows:

"The $j$ -invariant can be defined as a function on the upper half-plane $H = {$ τ ∈ C, Im(τ) > 0},

$j(\tau )=1728{\frac {g_{2}(\tau )^{3}}{\Delta (\tau )}}=1728{\frac {g_{2}(\tau )^{3}}{g_{2}(\tau )^{3}-27g_{3}(\tau )^{2}}}=1728{\frac {g_{2}(\tau )^{3}}{(2\pi )^{12}\,\eta ^{24}(\tau )}}$

with the third definition implying $j(\tau )$ can be expressed as a cube, also since 1728 ${}=12^{3}$ ."

But it is not at all clear what "expressed as a cube" means.

A cube of what ???

If 1728 were not the cube of an integer but instead were replaced by b³ for some non-integer complex number b, then of course we could still express the j-invariant as a cube ... of something.

I hope someone knowledgeable about this subject can fix this and make it clear what this means.