Anti Knight Sudoku (PuzzleMadness)

About Anti Knight Sudoku (PuzzleMadness)

The ordinary sudoku rules hold, with 1 to 9 once in every row, column and 3x3 box. The extra rule comes from chess. A knight moves two squares vertically and one horizontally, or two horizontally and one vertically, and in Anti Knight Sudoku no digit may appear again on any square a knight could reach from it. A 5 in the middle of the grid, for example, bars 5 from the eight cells around it in that L-shaped pattern.

Nothing extra is drawn on the grid, so the rule has to be held in your head. Its effect is strongest near the boxes' edges, where knight's moves reach into neighbouring boxes and can rule out the last places a digit could go. Today's Medium grid, for example, is a plain grid with purple given digits spread across all nine boxes.

The grid works like every sudoku on PuzzleMadness: number buttons above and below it, an eraser, and a pencil mode for candidate digits. On a keyboard, the number keys enter digits, the arrow keys move between cells, Ctrl, Shift or Alt with a number adds a pencil mark, and Delete or Backspace clears a cell.

PuzzleMadness is a free puzzle website run by Dewi, a software developer who started out making games for the PlayStation 2, Nintendo Wii and DS and now builds the site as a hobby. It carries dozens of puzzle types, from classic sudoku to newer logic grids, and each one gets its own dated puzzle every day at each difficulty. Anti Knight Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's Anti Knight Sudoku puzzle is shared by everyone who opens it.

The site checks your grid as you go and tells you when it is finished; it does not publish solutions. Each completed puzzle earns a score, and scores feed a monthly and an all-time Anti Knight Sudoku leaderboard, where only your best attempt at a puzzle counts. Older days stay in the archive, so a missed puzzle can still be played later and still counts toward the current month. Playing needs no account; a free registration gives you a username on the leaderboards and a cloud save. The pages carry adverts.

How to play Anti Knight Sudoku (PuzzleMadness)

1. Open today's Anti Knight Sudoku page on PuzzleMadness and choose a difficulty from the menu above the grid.
2. Click a cell and enter a digit with the number buttons above and below the grid or with the keyboard; the eraser button clears a cell.
3. Fill every row, column and 3x3 box with the digits 1 to 9, each once.
4. Never place a digit a knight's move (two cells in one direction and one to the side) away from the same digit.
5. Turn on pencil mode (the pencil button, or Ctrl, Shift or Alt with a number key) to jot candidate digits in a cell.
6. The site tells you when the grid is complete and adds your score; registered players appear on the leaderboards.

Screenshots

Tips

• When a digit has two possible places in a box, check which one is a knight's move from the same digit next door.
• Knight's moves always cross into a different row and column, so the rule mostly matters between neighbouring boxes.
• Scan one digit at a time across the whole grid; the knight rule hits a single digit hardest.
• Missed a day? Earlier puzzles are in the archive, and finishing one still counts toward this month's leaderboard.

Questions