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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.
Get started now by creating a new collection.
Read the kata's description again about what a "cubic" number is. Not a kata issue.
Thanks a lot man. You're absolutely correct. That rule is what make this kata different to his similiar kata: Step in Primes.
I don't get why people downvote your explanation. I was stuck because I didn't knew about this rule beforehand.