# Talk:Pell number

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Mathematics rating:
 B+ Class
 Mid Importance
Field: Number theory (historical)

## Nonsense formula

I moved the following claim to the talk page:

The closed form is:
$\pi = (1+\sqrt{2})^n - (1-\sqrt{2})^n\over\sqrt{8}$

This formula (if correct), needs to mention $P_n$ somewhere. As it stands, it doesn't seem to make any sense. Haseldon

It may be that this page could be split into two, or just explained differently.

There is more detail on this and I will do my best although my mathematics knowledge is not advanced at the moment. I think there is a closed form. Also there are some applications (see the references).

## New Section

Since Spring 2005 has passed, I ventured to add a new section that provides some examples and explains, rather than merely states, various formulas and asymptotic expressions. I'd like to beef it up a bit to replace the rest of the article but I thought I'd wait a while for comments before blanking out all those sections. I'm also going to link from the articles Triangular square numbers silver ratio and a few others where certain (correct but) unmotivated and out of the blue statements are made. --Gentlemath (talk) 00:28, 8 March 2009 (UTC)

I have to admit that right now it greatly resembles a big undigested mass of original research. I'm not going to immediately revert you, but it would be good if (1) you could provide sources rather than giving the appearance that you calculated all this yourself, and (2) you could provide more textual description to link and motivate the calculations, and integrate it better with the pre-existing sections of the article rather than making a separate section that duplicates some of the pre-existing content. I am a bit put off by your talk of blanking the pre-existing sections; if I had to choose only one of what was there beforehand or your new additions, I would choose the former, and I would very much not want to see the text and the referencing from the existing content go away. —David Eppstein (talk) 01:33, 8 March 2009 (UTC)

Thanks for the comments. Sorry, I suppose blanking was not the word I wanted. Note that I didn't disturb anything. I should have said: I'd like to beef it up a bit to replace much of the article but I thought I'd wait a while for comments before starting to in any way alter those sections. OK I did calculate it myself but I consider the calculations routine (others might differ). I read original research and it makes an allowance for routine calculations provided editors agree that the arithmetic and its application are correct. Only the very last line I put in is not simple calculation. I have not read Albert H. Beiler's delightful book recreations in the theory of numbers in about 30 years but he probably does things in about that way (I can check). I agree that the references show nice depth and would not want to disturb them unless it was to improve them. For example Martin, The Analyst Des Moines 1876 is a strange reference for a fact that could as well be J. Ozanam Récréations mathématiques et physiques (1694) ( first English edition around 1801). just for fun: http://cnum.cnam.fr/CGI/fpage.cgi?8PY9.1/70/100/506/441/460 I did make up the name Half companion Pell numbers since the companion Pell numbers are the favored Wikipedia name for double the numerators of the convergents to sqrt(2). The article now says

[here is an] identity of Pell numbers:
$\bigl((P_{k-1}+P_k)\cdot P_k\bigr)^2 = \frac{(P_{k-1}+P_k)^2\cdot\left((P_{k-1}+P_k)^2-(-1)^k\right)}{2}.$
and somehow that gives all the triangular square numbers.

Actually the article on square triangular numbers does the same calculations as me (not a coincidence..it is the obvious thing to do) and then passes off to Pell Equations. But this first and simplest of Pell equations is easily treated on its own giving a self contained treatment. If I do start integrating things in I am ready to be mercilessly edited but if it is obviously going to be unpopular then I won't. --Gentlemath (talk) 07:41, 8 March 2009 (UTC)