Chess Problem: What is the minimum number of white pieces required such that the black king is in check, no matter it's position?

Question

I can think of a solution with 7 pieces required. 7 Queens on a diagonal with one missing in the corner. Is there a solution which requires less pieces than this? If so, does it generalise to an nxn board?

Answer 1

Because queens attack the most squares, any problem where you are looking for the minimum number of nondistinct pieces is looking for the minimum number of queens attacking every square on the board. This link has a good description of what work has been done on the problem, but it does not seem to have a closed form generalized solution at the moment:

http://mathworld.wolfram.com/QueensProblem.html