Talk:Square pyramidal number

Square pyramidal number has been listed as one of the Mathematics good articles under the good article criteria. If you can improve it further, please do so. If it no longer meets these criteria, you can reassess it.
Review: December 25, 2021. (Reviewed version).

A fact from Square pyramidal number appeared on Wikipedia's Main Page in the Did you know column on 14 January 2022 (check views). The text of the entry was as follows:

Did you know... that the number of cannonballs in a square pyramid (pictured) with $n$ cannonballs along each edge is ${\frac {n(n+1)(2n+1)}{6}}$ ?

A record of the entry may be seen at Wikipedia:Recent additions/2022/January. The nomination discussion and review may be seen at Template:Did you know nominations/Square pyramidal number.

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Untitled

In all articles found the formula for the square pyramidal number is proofed by using the inductive methode. Isn't there a direct proof? Jon van den Helder

I thought inductive was direct. An empirical proof (e.g., having a computer calculate the first thousand values using each formula and then having it compare that the lists are all the same) would convince more people, but symbological snobs would look down on it. Anton Mravcek 17:49, 14 July 2006 (UTC)[reply]

This article could use some diagrams to illustrate how the numbers are "built" as pyramids. — Gwalla | Talk 22:28, 10 February 2007 (UTC)[reply]

I tried making one at Image:Square pyramidal number.svg but my radial gradiants are not looking correct. I'll try to find out what's wrong with the svg coding, but it shouldn't be linked onto the actual article until it looks better. —David Eppstein 23:04, 10 February 2007 (UTC) Done! —David Eppstein 23:30, 10 February 2007 (UTC)[reply]

Nice work! Thanks! — Gwalla | Talk 18:56, 11 February 2007 (UTC)[reply]

name

what's wrong with pyramid number? —Preceding unsigned comment added by 208.2.172.2 (talk) 01:05, 3 July 2009 (UTC)[reply]

infinite sum

The infinite sum of Tetrahedral numbers reciprocals $\scriptstyle \sum _{n=1}^{\infty }{\frac {6}{n(n+1)(n+2)}}={\frac {3}{2}}$ .
What will be the infinite sum of Square pyramidal number? 46.115.58.82 (talk) 23:49, 12 October 2012 (UTC)[reply]

Try expanding using partial fractions. Ozob (talk) 14:45, 13 October 2012 (UTC)[reply]

Expansion

I have an immense amount of experimentally discovered things I could add to "Squares in a Square" which involve different grid shapes and altogether different shapes such as triangles. So much so that I could very easily create a whole other Wikipedia article on it. I would enjoy doing so, but I thought I would get a second opinion. In addition, although my stuff is definitely true, as I said, I figured out this stuff experimentally, so I have nothing to reference but myself. I know that isn't a very good answer, but I could really improve the depth of understanding and knowledge of that topic. I hope you will comment. Frivolous Consultant (talk) 22:56, 17 October 2012 (UTC)[reply]

You need to find a published source for this material if you want to include it here; see Wikipedia:Verifiability and Wikipedia:No original research. In addition, if the solution doesn't involve the same sequence of numbers, it may be a bit off-topic for this article, because this article is not about square-in-square type problems, it's about the sequence of numbers. That doesn't mean it can't be mentioned at all (after all, we do mention the problem of rectangles in a square grid) but it means that it would probably be inappropriate to include much detail. —David Eppstein (talk) 23:01, 17 October 2012 (UTC)[reply]

I sort-of expected this answer, but I don't know what to do. People don't care enough about this topic, so no one makes anything about it. I've searched all over the Internet, but I haven't found anything. Although I haven't searched any books, I wouldn't know where to start, and besides, I would be exceptionally surprised if they had anything of value in them. If anyone can find anything, I'll defintely cite it, but as of now, I have nothing. I've thought about creating a formal website so that I have something to source, but I don't know how to do that. This isn't the first time something like this has happened to me before either. I could expand on that, but I'm already taking up too much space. I could add a formula to the "Squares in a square" part to improve it that wouldn't take up too much space and go off topic, but it still wouldn't have a reference. If you ask, I could figure out how to put it on here so you can look it over. That would be only a measely fraction of the unreferenceable knowledge I have on the topic. Also, I was suggesting creating a whole other page on Wikipedia for putting the rest and/or moving "Squares in a square" there, not adding a bunch to this article. I tried putting the suggestion on Wikipedia: Articles for creation today, but I couldn't get it to show up. I would appreciate help. Frivolous Consultant (talk) 00:20, 19 October 2012 (UTC)[reply]

Have you tried the Online Encyclopedia of Integer Sequences? You should be able to generate some sequences of numbers from your results and plug them into OEIS. I think it's pretty likely that you'll find some of your results there. You may also find references, maybe enough to write an article. Ozob (talk) 01:50, 21 October 2012 (UTC)[reply]

That website helped a lot. There is so much information on there about what I was looking for that it gave me a slight inferiority complex because of how many hours and hours I spent working on this stuff when it took me under a second to get a full sequence along with formulas. To my relief though, there was a few things that it didn't know, so I don't feel as bad, but it knows enough to give a good resorce. Be prepared for a new article on the subject, but there are two more things. First, I don't know what I should title it; I've recently been refering to the topic as tessellation conglomerates for lack of a better term, but that name is completely made up by me. Also, when I finish, it might be good to move "Squares in a square" to the page. (I realize what I've been typing takes up a lot of room. I won't be offended if you delete my previous entries.) Frivolous Consultant (talk) 23:21, 25 October 2012 (UTC)[reply]

If people "don't care enough about this topic" it may not be appropriate for wikipedia, meaning not notable--345Kai (talk) 03:12, 23 June 2015 (UTC)[reply]

Quadrature of the Parabola with the "square pyramidal number"

I found that the "square pyramidal number" can be used to prove the Archimedes' theorem on the area of parabolic segment. The proof, carried out without the use of "mathematical analysis", is readable at the following web adress: https://drive.google.com/file/d/0B4iaQ-gBYTaJMDJFd2FFbkU2TU0/view?usp=sharing

Sum of the first n squares (geometrical proof)

See at: https://sites.google.com/site/leggendoarchimede — Preceding unsigned comment added by Ancora Luciano (talk • contribs) 13:06, 27 June 2013 (UTC)[reply]

We represent the square pyramidal number P₆ = 91 with cubes of unit volume, as shown, and inscribe in building a pyramid (in red). Let V₆ the volume of the inscribed pyramid. To obtain P₆ you may add to V₆ the excess external volume to the red pyramid. Such excess is: 2/3 for each cube placed on the central edge, and 1/2 for the cubes forming the steps of the building (enlarge for a better look of highlighted part). Then, calculating one has:

P₆ = V₆+(2/3)*6+(1+2+3+4+5)

For the induction principle, will be:

P_n = V_n+(2n)/3+Σ_n(from 1 to n-1)n

P_n = n³/3+2n/3+(n²+n)/2-n

P_n = (2n³+3n²+n)/6

Geometric representation of the square pyramidal number using cubes, instead of sferes, is more useful.

George Polya, in his book "Mathematical discovery", Volume 2, 1968, presents this solution saying it "rained from the sky", as obtained algebraically with a trick, like a rabbit drawn out from the hat. Returning to the introduction to this talk page, seems that this (realistic) geometrical derivation is a little more direct than the algebraic presented in the article.

In the article "Summation" was added the talk: "Sum of the first n cubes (geometrical proof)", please to see it. --Ancora Luciano (talk) 06:15, 22 May 2013 (UTC)[reply]

Semi-protected edit request on 30 December 2013

This edit request has been answered. Set the |answered= or |ans= parameter to no to reactivate your request.

Something seems amiss in the sentence starting "Now there". I suggest replacing "Now there" with "There are". Bill01568 (talk) 16:29, 30 December 2013 (UTC)[reply]

Seems reasonable,

Done. Little Mountain 5 22:35, 30 December 2013 (UTC)[reply]

Simplification of the current proof

It seems that current proof given in the article could be simplified by using $1^{2}+2^{2}+\ldots +n^{2}=1+2+2+3+3+3+\ldots +n+n+\ldots +n$ instead of $1^{2}+2^{2}+\ldots +n^{2}=(2n-1)+(2n-3)+(2n-3)+\ldots +1+1+\ldots +1$ , with sum of the column still being $2n+1$ . Unfortunately, I really can't be bothered to find citations for this proof. Fortunately, there are no citations for the current proof as well, so changing it won't make it worse. Opinions? MYXOMOPbI4 (talk) 03:51, 7 September 2017 (UTC)[reply]

I'm not sure I see the point of long algebraic derivations at all, given that it's so straightforward to prove any such formula by induction. If you could replace this with a short conceptual visual (and sourced!) proof, that would be better, I think. —David Eppstein (talk) 04:06, 7 September 2017 (UTC)[reply]

The proof by induction is indeed not that hard, but you need to know the formula in the first place. This way you get the formula automatically, and the proof isn't long at all (it can be done with one picture, http://forumbgz.ru/user/upload/file580638.jpg (with some unimportant text in russian)). But I'm not sure if some random picture on the internet is considered a source. MYXOMOPbI4 (talk) 22:53, 7 September 2017 (UTC)[reply]

It's more or less obvious that the formula is a cubic polynomial and from that and the first four values you can immediately derive it by finite differences. And no, random pictures on the internet, especially from a site whose address syntax suggests that it's an open wiki, are not reliable sources. —David Eppstein (talk) 00:27, 8 September 2017 (UTC)[reply]

Link addition

A link to Lucas Numbers should be provided for completeness. 199.209.255.246 (talk) 14:40, 10 September 2018 (UTC)[reply]

Only if the connection can be sourced. Otherwise the unsourced paragraph mentioning Lucas should be removed. —David Eppstein (talk) 16:33, 10 September 2018 (UTC)[reply]

Archimedes

I don't see any mention of Archimedes, who probably gave the first formula for the sum of squares of the first n natural numbers in his book 'On Spirals'. It doesn't at first sight look like the modern formula, and the derivation is horribly complicated, but it is there after all. See the discussion in Heath's edition of Archimedes, especially on page 109 in the Dover edition.2A00:23C8:7906:1301:A453:48F9:36D5:B594 (talk) 21:23, 7 March 2021 (UTC)[reply]

Added. I used the 1897 edition rather than the Dover edition, but the pagination is the same. —David Eppstein (talk) 00:43, 8 March 2021 (UTC)[reply]

GA Review

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

This review is transcluded from Talk:Square pyramidal number/GA1.

Reviewer: Olivaw-Daneel (talk · contribs) 01:45, 18 December 2021 (UTC)[reply]

Good Article review progress box

Criteria: 1a. prose () 1b. MoS () 2a. ref layout () 2b. cites WP:RS () 2c. no WP:OR () 2d. no WP:CV ()

3a. broadness () 3b. focus () 4. neutral () 5. stable () 6a. free or tagged images () 6b. pics relevant ()

Note: this represents where the article stands relative to the Good Article criteria. Criteria marked are unassessed

Interested in reviewing this. I'll have comments up in a couple days. Olivaw-Daneel (talk) 01:45, 18 December 2021 (UTC)[reply]

The article is well-illustrated, but I think it covers the material a bit too tersely – some further elucidation would greatly help. Specific suggestions below.

Lead

Currently the first sentence requires the reader to know what a figurate number is – suggest postponing the mention of figurate numbers to a separate sentence that hints at their definition. (Perhaps something like "It can be generalized to a broader category of numbers that are known as figurate numbers...")
- Added a gloss for figurate numbers. —David Eppstein (talk) 07:20, 22 December 2021 (UTC)[reply]

Formula

There seem to be two competing definitions here – algebraic and geometric – and I'm not sure which of them is more primary. Since the lead starts out with the geometric version, suggest doing the same here. (It may also be logical to rename this section to Definition).
- It really was intended as a formula solving the question "how many points are in a pyramid" rather than a definition "these numbers are the values of a cubic polynomial". I rewrote to suggest that ordering more strongly. —David Eppstein (talk) 07:26, 22 December 2021 (UTC)[reply]
  - I like the new version. A few comments: the sphere interpretation is not mentioned in ref #1 (suggest adding ref #10 Beiler); numbers of points should be numbers of spheres; height probably refers to the number of layers but could be clearer.
    - Ok, height is now number of layers, and the sphere interpretation cited to Beiler is used more consistently until the Ehrhart paragraph. —David Eppstein (talk) 19:20, 24 December 2021 (UTC)[reply]
The lead hints at a historical perspective – The study of these numbers goes back to Archimedes and Fibonacci – that one would expect to be covered here. (E.g. When and for what purpose this was studied.)
- Turns out Archimedes and Fibonacci had different purposes than counting points in pyramids. Added. —David Eppstein (talk) 07:26, 22 December 2021 (UTC)[reply]
The mention of figurate numbers appears all of a sudden; as in the lead, it would be helpful to gloss them before use.
- Added another gloss, also using Beiler to source the jump from spheres to points. —David Eppstein (talk) 19:20, 24 December 2021 (UTC)[reply]
The more general statements about Erhart polynomials are uncited (ref #5 Beck only talks about polytopes with integer vertices)
- Rewritten to only talk about Ehrhart for integer polyhedra, per the source. —David Eppstein (talk) 19:20, 24 December 2021 (UTC)[reply]

Geometric enumeration

It would be helpful to add a prefatory remark similar to this one from the lead: As well as counting spheres in a pyramid, these numbers can be used to solve several other counting problems
- Done —David Eppstein (talk) 19:43, 24 December 2021 (UTC)[reply]
Square counting: the refs don't mention square pyramidal numbers (suggest adding ref #1 Sloane)
- Done —David Eppstein (talk) 19:43, 24 December 2021 (UTC)[reply]
Rectangle counting: this statement seems related to the square counting problem, so it should probably be moved up.
- Done —David Eppstein (talk) 19:43, 24 December 2021 (UTC)[reply]

Relations to other figurate numbers

In the first sentence, it should be made more clear that the pyramid is made of cannonballs.
- Reordered to put the cannonballs earlier. —David Eppstein (talk) 19:45, 24 December 2021 (UTC)[reply]
This fact was proven by G. N. Watson in 1918 – I'm not sure if Watson deserves sole credit. Ref #9 Anglin says that he filled gaps in earlier drafts by Moret-Blanc (1876) and Lucas (1877).
- Added a mention to earlier incomplete proofs by Moret-Blanc and Lucas. —David Eppstein (talk) 21:02, 24 December 2021 (UTC)[reply]
The sum of binomials equation is not cited (suggest adding ref #11 Caglayan)
- Since Caglayan phrases this as a problem rather than a statement, I used two other references, Conway & Guy's Book of Numbers and a paper by Grassl. Grassl explicitly uses the binomial coefficient description of this formula rather than calling them tetrahedral numbers as the other sources do. —David Eppstein (talk) 22:43, 24 December 2021 (UTC)[reply]
In ref #10 Beiler: pp. 194 → pp. 194–195
- Ok. —David Eppstein (talk) 21:11, 24 December 2021 (UTC)[reply]
Last paragraph: a single tetrahedron of slightly more than twice the edge length – why is the length slightly more than twice? (The source says: "Four times the nth square pyramidal number is the (2n)th tetrahedral number".)
- The edge length of $P_{n}$ $P_{n}$ is $n-1$ $n-1$ . The edge length of $4P_{n}=T_{2n}$ $4P_{n}=T_{2n}$ is $2n-1=2(n-1)+1$ $2n-1=2(n-1)+1$ , slightly more than twice. Edge length is off by one from the index in the sequence of these numbers. —David Eppstein (talk) 22:10, 24 December 2021 (UTC)[reply]
  - Ah I see, so that's based on an "edge" as a line connecting the centers of the spheres – do sources define it this way? It seems at odds with the last figure which says ⁠ $P_{24}$ ⁠ has side 24. Olivaw-Daneel (talk) 00:01, 25 December 2021 (UTC)[reply]

Other properties

Suggest expanding a little to avoid a single-sentence section. Some possibilities: the connection to Archimedes' approximation for π (22/7) mentioned in the ref; some background on the Leibniz formula.
- I found a connection to approximation theory and added it. While searching for material for this expansion, I also found additional material on the history of these numbers (now split off into a separate history section) and on using them to count certain integer matrices (added to the end of the enumeration section). —David Eppstein (talk) 23:24, 24 December 2021 (UTC)[reply]

Placing it on hold; please ping when you're done. Thanks. Olivaw-Daneel (talk) 09:47, 24 December 2021 (UTC)[reply]

@Olivaw-Daneel: All comments addressed; I think it's now ready for a second look. —David Eppstein (talk) 23:24, 24 December 2021 (UTC)[reply]

Just one last comment above about edge length. The article looks great; I really like the history section. Olivaw-Daneel (talk) 00:01, 25 December 2021 (UTC)[reply]

Ok, changed to "points along each edge" rather than edge length to avoid that ambiguity. —David Eppstein (talk) 00:11, 25 December 2021 (UTC)[reply]

Great, congrats on the GA. Olivaw-Daneel (talk) 00:23, 25 December 2021 (UTC)[reply]

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Did you know nomination

The following is an archived discussion of the DYK nomination of the article below. Please do not modify this page. Subsequent comments should be made on the appropriate discussion page (such as this nomination's talk page, the article's talk page or Wikipedia talk:Did you know), unless there is consensus to re-open the discussion at this page. No further edits should be made to this page.

The result was: promoted by Kavyansh.Singh (talk) 12:12, 9 January 2022 (UTC)[reply]

(

Comment or view
Article history

)

Square pyramid of cannonballs

... that a square pyramid of cannonballs (pictured) with n cannonballs along each edge of the square has a total of n(n + 1)(2n + 1)/6 balls, a square pyramidal number, in it? Source: https://oeis.org/A000330
- ALT1: ... that both the Greeks and the Japanese counted balls stacked into pyramids (pictured)? Source: Greeks: Federico, https://doi.org/10.1007/978-1-4612-5759-2; Japanese: Yanagihara, https://www.jstage.jst.go.jp/article/tmj1911/14/0/14_0_305/_pdf
- ALT2: ... that a single formula can be used to count balls stacked into pyramids (pictured), squares of all sizes in a chessboard, and acute triangles in an odd regular polygon? Source: https://oeis.org/A000330
- Reviewed: Template:Did you know nominations/Ken Lyotier

Improved to Good Article status by David Eppstein (talk). Self-nominated at 00:47, 27 December 2021 (UTC).[reply]

Congratulations on the GA promotion, David Eppstein! The article is in great shape, and it's about a very important topic. We used it as an early example when teaching induction, so having a good resource on Wikipedia is helpful for those who want to immediately learn more about the idea.
The article was nominated on time and is long enough; no close paraphrasing found; NPOV and well-cited; hooks are cited, AGF with the offline sources in the article. I find all of the hooks broadly appealing and interesting, although the lengths of ALT0 & 2 may turn some people off. The image is appropriately licensed, relevant, and clear at this resolution. QPQ done. I'll leave the choice of hook to the promoter, since while the lengths of ALT0 & 2 are not a problem for me, it's a judgment call. Again, good work, and congratulations. Urve (talk) 04:44, 30 December 2021 (UTC)[reply]
- @Urve: Thanks! In case length rather than the technicality of the formula is a concern, here's a tighter version of ALT0:
- ALT0b: that the number of cannonballs in a square pyramid (pictured) with $n$ cannonballs along each edge is $n (n + 1)(2 n + 1)/6$ ?
—David Eppstein (talk) 07:13, 30 December 2021 (UTC)[reply]
That works! I don't think the formula is a problem - in fact, it may draw some more eyes to the article, because we rarely have formulas on the front page. My preference is for ALT0b in this case, but as always, it's a matter of taste, so leaving to the promoter. Urve (talk) 07:26, 30 December 2021 (UTC)[reply]

Promoting ALT0b to Prep 5, with the image – Kavyansh.Singh (talk) 12:12, 9 January 2022 (UTC)[reply]