# Record of OEIS on PPCG

The OEIS is a great resource, and a lot of challenges on PPCG are based on or around sequences that are included on the OEIS.

I thought it might be a good idea to compile a list of sequences that have challenges. This could be useful for a number of reasons.

• You could use the list to check if a question related to an OEIS sequence is a dupe.

• If you want a question idea, you can use the list to find potentially interesting OEIS sequences that don't have questions.

• If you like a sequence you could see what questions have been asked about it.

Where might such a list live? We could compile it in an answer to this question, or using an external tool.

• Don't forget challenges that have spawned OEIS entries. We have a few of those :) – Calvin's Hobbies Jul 6 '17 at 4:42
• Once we have an updated list we can order it by question ID, and submit the resulting list of integers to OEIS. Then somebody will open a challenge regarding such sequence and we'll come full circle. – Matteo Italia Jul 10 '17 at 20:09
• You could filter the SE API for all entries with content.contains("oeis.org") and scrape the results. – Magic Octopus Urn Jul 11 '17 at 22:45

When a question is about an OEIS sequence, it should include the name of the sequence (Axxxxxx) and a link to OEIS in the question body. When a question turns out to correspond to a sequence, the name and link should be edited into the question body. That deals with your three points:

• You could use the list to check if a question related to an OEIS sequence is a dupe.

Search for the name of the sequence.

• If you want a question idea, you can use the list to find potentially interesting OEIS sequences that don't have questions.

That's really the same point again, because finding potentially interesting sequences among all the not very interesting ones is the hard part.

• If you like a sequence you could see what questions have been asked about it.

That's certainly the same point again.

If there is an additional need to generate a list of sequences which have been used, I think that should be possible as a query on data.stackexchange.com, although I'm a bit rusty on string handling in T-SQL so I won't attempt it now.

This does also imply that we might need a site maintenance project to go through looking for questions which have links to OEIS in the comments but not in the question body. If you decide to do that, please bear in mind that edits bump questions, so with a project like this it's best to only edit about 5 questions a day.

I've made something like what you've requested. It's online at http://ppcgoeis.surge.sh.

I wrote a small bit of JS to automatically create a JSON file with all the challenges available here

Essentially, it just goes through challenges that contain OEIS, then finds anything that match the regex A\d{6}. Then, it categorises it by checking the tags: If it's , it classifies it as Check. If it's , it's categorised by nth term. Otherwise, classify it as Unknown type. This seems to work for most challenges, but of course there will be some inaccuracies.

Please note that I am still working on this and it may be buggy.

• Perhaps change your regex to A\d{6} ? – Pavel Jul 6 '17 at 2:18
• @Phoenix I've changed that, and updated the entries. – Okx Jul 9 '17 at 18:55
• Bug report: codegolf.stackexchange.com/q/51266/194 should definitely be in the list at least once, but isn't – Peter Taylor Jul 9 '17 at 21:43
• To follow on what @PeterTaylor mentioned, this tool only searches question bodies when it should search titles and comments too; very often people will comment the OEIS link but it does not make it into the question – cat Jul 16 '17 at 18:07

There's no specific tag or list for OEIS challenges. The closest would be searching the tag (or searching both and ), or searching for the term "OEIS" in questions. We previously had a proposal to add an OEIS tag, but it was decided against.

I don't really think the site would benefit from having a list of OEIS challenges. For starters, we'd have to maintain it, and so it will inevitably be out of date. Additionally, I don't think it would provide any additional utility over simply searching challenges.

• I don't think that searching the sequences tag is nearly as helpful as a list of OEIS problems. I already find it pretty hard to find if a sequence has been asked about because people don't generally put the OEIS number in the body of their post. I would also like to point out that not all challenges related to an OEIS challenge are sequence challenges, some of them are decision-problems. I don't think an OEIS tag would help either and I am against its creation. – Ad Hoc Garf Hunter Jul 3 '17 at 4:24
• I intend to make this list whether or not you think it would be helpful, because I think it would be helpful. I wanted to know where I might put it to be most convenient for other people to use. – Ad Hoc Garf Hunter Jul 3 '17 at 4:25

In my personal opinion, the best way to accomplish this would be to do one of the following:

• Allow OEIS tags like then use the SE API to find them.
• Create a standard in titles like: [A000001] {TITLE CONTENT}

Personally, I think the first wouldn't be too bad, unless you're trying to search for tags using just 'a'. Maybe even using a different tag convention like OEIS000001 would be better?

# Create oeis tag and use it

Create an tag, and add this tag to such questions.

Any questions with this tag should contain the sequence id (Axxxxxx), and maybe a link, in question body.