## Cross flips

### Problem 331

Published on Sunday, 3rd April 2011, 08:00 am; Solved by 247`N``N` disks are placed on a square game board. Each disk has a black side and white side.

At each turn, you may choose a disk and flip all the disks in the same row and the same column as this disk: thus 2`N`-1 disks are flipped. The game ends when all disks show their white side. The following example shows a game on a 55 board.

It can be proven that 3 is the minimal number of turns to finish this game.

The bottom left disk on the `N``N` board has coordinates (0,0);

the bottom right disk has coordinates (`N`-1,0) and the top left disk has coordinates (0,`N`-1).

Let C_{N} be the following configuration of a board with `N``N` disks:

A disk at (`x`,`y`) satisfying , shows its black side; otherwise, it shows its white side. C_{5} is shown above.

Let T(`N`) be the minimal number of turns to finish a game starting from configuration C_{N} or 0 if configuration C_{N} is unsolvable.

We have shown that T(5)=3. You are also given that T(10)=29 and T(1 000)=395253.

Find .