Maybe a second example in the description showing that would help.

Oh sorry, I believed only the largest numbers are available to remove.

Not a kata issue, remove 1 and 3. And the same question was answered two posts below yours.

I think, I found an error on C# tests. The array is [9, 13, 21, 1, 3, 5, 7], the smallest number is 1, all squares roots the other numbers are bigger than 1, except of course 1. Six numbers are required to be removed to achieve the goal, but the unit test shows that only 2 are needed. Is an error? or I am doing something wrong?

I think unit tests are wrong.

I think this report has not enough details to fix the tests.

I think C# unit tests are wrong