Question 1

What is Diagonal Sudoku?

Accepted Answer

Both main diagonals, corner to corner, must also hold each digit exactly once, just as a row or a column does. Two extra regions, drawn at an angle, for the student who found rows too orthogonal.

Question 2

How do you solve Diagonal Sudoku?

Accepted Answer

Each diagonal holds four cells. With the ordinary rules and the diagonal rule together, the middle of the grid becomes very crowded indeed. A cell on a diagonal belongs to four regions at once: its row, its column, its box and the diagonal. Those are the most constrained cells on the board, so solve them first. A digit already on a diagonal cannot appear on it again. Obvious, yes. It is also the rule most often forgotten halfway through. The two diagonals cross at the centre. On a 6 by 6 grid the central cells, R3C3, R3C4, R4C3 and R4C4, sit on or beside both of them. A diagonal cell that also lies in a central box is the most constrained cell you will find. Give it your attention early. Only certain cells are affected: those where the row equals the column, which is the main diagonal, and those where row plus column equals the size minus one, which is the anti-diagonal. If your cell lies on neither, this rule has nothing to say to it. When scanning for a digit, treat each diagonal as one more region to eliminate from. It will catch placements you would otherwise walk straight past.

Diagonal Sudoku

How do you solve Diagonal Sudoku?

Start on a 4 by 4 grid

Then try a 6 by 6 grid

What does Diagonal Sudoku look like?

Where can you play Diagonal Sudoku?