Every element of the sequence satisfies a condition that certain power (e.i. ANY) of the sum of its digits is equal to the number itself. Your calculations are correct but the test cases are fine as well ;)

This is neither clever nor best practice at all!! Evaluating condition "result.Count(i => i == item) < x" for each and every item in arr is most likely to be very expensive. Or are the upvotes just for shortness of the method??? I'm really upset seeing such a code that appreciated :(