Ninefold Sudoku

Nine rows, nine columns, nine boxes, and one arrangement that satisfies all of them at once. Each morning four new grids appear, the same for everyone, from a quiet warm-up to one that asks for a chain of reasoning ten steps long.

Sudoku in four levels graded by the techniques each puzzle needs, with pencil marks, undo and four daily puzzles that are the same for everyone.

Fill the 9x9 grid so every row, every column and every 3x3 box holds the digits 1 to 9 exactly once. Tap a cell, then tap a digit on the keypad; on a keyboard use the arrow keys and the number keys. Notes mode (the Notes button or N) writes small pencil marks instead, and a digit typed with Shift held does the opposite of the current mode. Placing a digit clears that digit from the notes in its row, column and box. Undo (or U, or Ctrl+Z) steps back one move, Erase (or Backspace) clears a cell. Two digits that clash in a row, column or box are always shown in red; turn on Check to also mark any digit that does not match the solution. Every puzzle has exactly one solution. Easy needs only singles, Medium adds pairs and pointing, Hard adds triples, X-wing and short chains, Expert needs long chains or swordfish. A game you leave is saved per level and continues where you stopped. The score grows with the level and the speed of the solve.