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Here is a list of active sites where you can find many unique and interesting math challenges and puzzles from math enthusiasts that collect and create puzzles the last decades
https://www.primepuzzles.net/
A new puzzle every week
https://www.shyamsundergupta.com/canyoufind.htm
One or two demanding puzzles per year by recreational mathematician Shyam Sunder Gupta
mathematics-amazing-fun-https-projecteuler-net-archives-One-greatest-projects-with-hundreds-of-puzzles-number-theory-combinat
https://projecteuler.net/archives
One of the greatest projects with hundreds of puzzles for number theory and combinatorics enthusiasts
https://www.janestreet.com/puzzles/archive/
A new puzzle every month
https://oeis.org/
This is the famous encyclopedia where extending some of the sequences can be turned into an amazing puzzle (just search "keyword:more")
https://research.ibm.com/haifa/ponderthis/index.shtml
One puzzle every month
http://azspcs.com/
One or two contests per year. Each contest asks that you come up with your best solutions to a set of computationally intensive problems, generally involving sophisticated heuristics
I hope you find this list interesting!
More people should learn about Recreational Maths
Join as in discord "Recreational Maths & math Puzzles" for more
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