No answer. Since TCAU says "good problem" I consider the issue as resolved.

Can we get some more details as to what is not working?

I improved the solution in other languages:-) Thank for the feedback!

As a comment on my solution I explained how this problem can be solved without going through the entire series from i = 1 to n, just by applying a limited amount of equations. This results in a solution which needs less calculation power (I got around 2300 ms when going through the series, and around 1400 ms with the 'optimized solution'.) As you gave the suggestion to optimize the solution, this problem would just get that extra edge, and increase the difficulty slightly, if such an optimized solution was actually needed in order to fulfill the kata.

I don't understand what you mean. Please can you be more explicit?

C# part doesn't work just yet. Good problem.

Thanks for your feedback!

Nevermind I got found out what the issue was. Thanks for your help!

It's not in the habit of Codewars to give the tests, sorry for that:-(
2nd test is absolutely ordinary, no edge case.

Meh it still won't work for some reason. Could I have the parameters used for the second test upon submission? Could be an edge case that I didn't cover yet.

I don't see anything special in 2nd test! Maybe Codewars was too busy, it happens:-(

There's a bug in the C# version.

Assert.AreEqual(Kata.Maskify("4556364607935616"), "############5616");

ExamplesTests
Expected string length 28 but was 16. Strings differ at index 12. //should only have 16 characters
Expected: "############4556364607935616" // should only have last four digits unmasked
But was: "############5616"
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