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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
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Surprising how few solutions add the optimization constraint of [2..sqrt x] when sieving for factors. I'd suggest updating the test cases to have a sufficiently large prime group (10000 seems reasonable) to force thinking about the characteristics of prime factorization.