Puzzle: An 8×8 matrix contains zeros and ones. You may repeatedly choose any 3×3 or 4×4 block and flip all bits in the block (that is, convert zeros to ones, and ones to zeros). Can you remove all the ones in the matrix?
Source: Gurmeet Singh Manku's Blog where he said that he saw it in an old Russian problem book.
Highlight the part between the * symbols for the answer.
* There are 36 3×3 blocks and 25 4×4 blocks. Given a sequence of blocks, the same effect is achieved no matter in what order the blocks are chosen. If the same block occurs two times in a sequence, their effect is nullified. Thus, a sequence of blocks is equivalent in effect to some subset of the 36 + 25 = 61 blocks. Starting with all zeros, at most 2^61 distinct configurations out of 2^64 possible configurations of the matrix can be reached. This means that we may not be able to remove all the ones from the original matrix. *