

Project Euler - snth
http://projecteuler.net/

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RiderOfGiraffes
Fantastic resource, and mentioned here many, many times. Here are some
previous submissions:

<http://news.ycombinator.com/item?id=1262968>

<http://news.ycombinator.com/item?id=203317>

<http://news.ycombinator.com/item?id=86365>

You can find many, many references with this search:

<http://searchyc.com/projecteuler?sort=by_date>

