Factor Line Sudoku (PuzzleMadness)

Every row, column and 3x3 box holds 1 to 9 once each, as usual. Each Factor line has a circle at one end, and every other cell on the line must contain a factor of the circled digit. Within 1 to 9 the factors are short lists: 1 has only 1, a prime such as 5 or 7 has 1 and itself, 4 has 1, 2 and 4, 6 has 1, 2, 3 and 6, 8 has 1, 2, 4 and 8, and 9 has 1, 3 and 9. A line need not use every factor, and the rules page lists them for reference.

Since a digit can only repeat on a line when the cells are in different rows, columns and boxes, a long line needs a circle with plenty of factors, which in practice means 6 or 8 in the circle and lots of 1s, 2s, 3s and 4s along the line. The digits 5 and 7 can only appear on a line whose circle holds the same digit, and such a line can only hold 1s and that same digit.

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. Factor Line Sudoku comes in Easy, Medium, Hard and Tough versions plus a Mixed page, and the same day's Factor Line 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 Factor Line 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.