Tag Archives: william stein

LMFDB!

Very happy to see that the L-functions and Modular Forms Database is now live!

When I was a kid we looked up our elliptic curves in Cremona’s tables, on paper.  Then William Stein created the Modular Forms Database (you can still go there but it doesn’t really work) and suddenly you could look at the q-expansions of cusp forms in whatever weight and level you wanted, up to the limits of what William had computed.

The LMFDB is a sort of massively souped up version of Cremona and Stein, put together by a team of dozens and dozens of number theorists, including too many friends of mine to name individually.  And it’s a lot more than what the title suggests:  the incredibly useful Jones-Roberts database of local fields is built in; there’s a database of genus 2 curves over Q with small conductor; it even has Maass forms!  I’ve been playing with it all night.  It’s like an adventure playground for number theorists.

One of my first trips through Stein’s database came when I was a postdoc and was thinking about Ljunggren’s equation y^2 + 1 = 2x^4.  This equation has a large solution, (13,239), which has to do with the classical identity

\pi/4 = 4\arctan(1/5) - \arctan(1/239).

It turns out, as I explain in an old survey paper, that the existence of such a large solution is “explained” by the presence of a certain weight-2 cuspform in level 1024 whose mod-5 Galois representation is reducible.

With the LMFDB, you can easily wander around looking for more such examples!  For instance, you can very easily ask the database for non-CM elliptic curves whose mod-7 Galois representation is nonsurjective.  Among those, you can find this handsome curve of conductor 1296, which has very large height relative to its conductor.  Applying the usual Frey curve trick you can turn this curve into the Diophantine oddity

3*48383^2 – (1915)^3 = 2^13.

Huh — I wonder whether people ever thought about this Diophantine problem, when can the difference between a cube and three times a square be a power of 2?  Of course they did!  I just Googled

48383 Diophantine

and found this paper of Stanley Rabinowitz from 1978, which finds all solutions to that problem, of which this one is the largest.

Now whether you can massage this into an arctan identity, that I don’t know!

 

 

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Licensing changes at the Online Encyclopedia of Integer Sequences

Neil Sloane’s invaluable OEIS has just launched its new page, featuring among other things a wiki and a backend that allows the encylopedia to grow with much less personal supervision from Sloane himself.  But not all the changes are welcome:  William Stein points out the new end-user licensing agreement, which is much more restrictive than previously.  Here’s what William posted to the number theory listserv:

…there are now new and very significant restrictions on using OEIS content:
“To make copies, distribute, make Adaptations and make copies of and
distribute the Adaptations, of no more than 5% of the OEIS Content”.
This is in sharp contrast to how OEIS was before, where, e.g., there
was a file  http://oeis.org/stripped.gz  that contained the sequences
themselves.  (This matters to me, since we make them available for
Sage, with a nice interface.)  This was an incredibly useful tool,
since it meant that even without internet access (or in a secure
closed network), one could do searches of OEIS, which is in my opinion
a critical research tool that was built partly by the effort of the
community (you).   Distribution of this stripped.gz file now appears
to be illegal.

I think the recent direction OEIS is going in is unfortunate.  It’s
the exact opposite of how, e.g., Wikipedia operates.  Anybody can
mirror Wikipedia content, there is a complete 6GB tarball you can
download that contains all article, etc.

So before signing up for an account, and signing away the rights to
all data or sequences you’ve submitted, please consider the above.

I will not be registering or contributing to OEIS until the license changes.

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