
The On-Line Encyclopedia of Integer Sequences - colinprince
http://oeis.org/
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jsomers
The way I think about it is less as an encyclopedia of sequences per se than
it is a kind of projection of mathematics onto sequence-space. The point being
that an integer sequence, because it often has so many mathematical
interpretations, acts in some sense like the _intersection_ of those
interpretations. There are of course many ways in which two or more
mathematical concepts are linked, but a shared integer sequence is among the
most useful, precisely because it can be browsed and searched like an
encyclopedia.

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Sven7
That's a pretty thought

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i0exception
I've used OEIS a number of times for obscure mathematical problems in
programming competitions. It's a pretty useful resource!

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Roritharr
Could you please elaborate to a less mathematically inclined dev how to spot a
problem the would fit the bill and then how to use this encyclopedia properly?

It's one of these things I seem to have been born blind towards.

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Terr_
Well, the last time it helped me was as a "reverse lookup" for math knowledge
I never got the "right" way through education.

For example, one of the Google-Foobar puzzles involves how many ways you can
order a line of different-height elements so that only a certain number are
"visible" if you stood at one end of the line or the other.

First I tried to figure out how I would solve it for small numbers by hand,
creating a spreadsheet of inputs. At one point, I got a grid of numbers where
the "simpler" rows/columns had a sequence like 1, 1, 1, 2, 3, 1, 6, 11, 6,
which, through random googling and OEIS turned out to be "Unsigned Stirling
numbers of the first kind"

[http://oeis.org/A008275](http://oeis.org/A008275)
[https://en.wikipedia.org/wiki/Stirling_numbers_of_the_first_...](https://en.wikipedia.org/wiki/Stirling_numbers_of_the_first_kind)

Alas, I'm still no math-major... the comments in my solution contain: "I'm
extremely proud of this boiled-down end-result... except that I'm not sure I
can fully explain _why_ it works."

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subnaught
Interesting profile of the founder of this site:
[http://www.theguardian.com/science/alexs-adventures-in-
numbe...](http://www.theguardian.com/science/alexs-adventures-in-
numberland/2014/oct/07/neil-sloane-the-man-who-loved-only-integer-sequences)

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Terr_
Speaking as a non-mathematician, it's one of those resources you didn't know
you needed until that one time you have some sequence of numbers and wanted to
figure out if they meant something special.

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viraptor
Like the very known sequence? ;)
[http://oeis.org/A104101](http://oeis.org/A104101)

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Terr_
Heh, no, but two that I did "find in the wild":

[http://oeis.org/A195581](http://oeis.org/A195581)

[http://oeis.org/A008275](http://oeis.org/A008275)

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tanderson92
One of the best OEIS sequences in terms of user comments is surely A000027
(the positive integers, sure to be filled with silly 'explanations'). My
favorites:

a(n) is also the number of permutations simultaneously avoiding 213, 231 and
321 in the classical sense which can be realized as labels on an increasing
strict binary tree with 2n-1 nodes. See A245904 for more information on
increasing strict binary trees. - Manda Riehl

Number of n-digit numbers the binary expansion of which contains one run of
1's. - Vladimir Shevelev

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madcaptenor
A000012 is the all-ones sequence:
[https://oeis.org/A000012](https://oeis.org/A000012). It even has a subreddit:
[https://www.reddit.com/r/A000012/](https://www.reddit.com/r/A000012/)

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rsc
Written in Go. :-)

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viraptor
Where did you find that? The service existed a long time before Go was
created.

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lifthrasiir
That's because Russ Cox (rsc) is the very person who has rewritten a decade-
old software for OEIS, as seen in the OEIS foundation page:

> It took us over a year to resolve this problem. In the end, Russ Cox
> completely rewrote all the programs needed to maintain the database and
> answer queries - a huge task! NJAS's colleague David Applegate has also been
> of enormous help in getting the new system working.

([http://oeisf.org/index.html#HISTORY](http://oeisf.org/index.html#HISTORY))

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chameleonSkin
My favorite sequence I learned from this:

Decimal expansion of (7^(e - 1/e) - 9)*Pi^2, also known as Jenny's constant.

