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       Sudoku is a fascinating game of pure logic that involves no math
whatsoever -- and no guessing. Using the given numbers as guides, your job
is to fill in the grid so that every row, every column, and every 3x3 framed box
contains the numbers 1-9 exactly once. The solution to each puzzle is unique.

To enter a final number, click anywhere inside the square above the short line and type in the number. To keep track of "possible numbers" for a square, click anywhere below the short line and type in the numbers. NOTE: It is not necessary to "erase" (or remove) the little possible numbers as you proceed -- leaving them or "erasing" them will not affect the outcome of the puzzle.
To toggle left to right from square to square, hit the tab key.

       (1) Box-Stripping: Look at any set of three side-by-side boxes (or subregions), either across or down, and see if two of the three contain an identical number while the third box in the set doesn't. If there are two that do contain the same number, notice how the rows (or columns) that contain these two numbers severely limit where that number can go in the remaining box. Pre-existing numbers may narrow down the placement even more.
       (2) Finishing off: Pick a row, column, or box that's as close to being completed as possible, determine what the remaining numbers have to be, and start cross-checking -- including the box.
       (3) Twins and triplets: If you have two identical possibles (such as 4,5 and 4,5) in a single row, column, or box, neither number can appear anywhere else in that row, column, or box. Similarly, if you have three numbers as possibles in exactly three squares (such as 2,6 and 2,6,9 and 6,9) in a single row, column, or box, none of those numbers can appear anywhere else in that row, column, or box.
       (4) Box-Flushing: Eraser-Free Sudoku puzzles never require guessing, but our hard puzzles often require this one other strategy. Suppose that a row or column contains two squares with the possibles 3,7 and 3,8,9. Suppose also that both of these squares happen to occur within a single box. If you know for sure that there are no other 3's in the rest of that row or column, then one of those 3's has to be the final 3 for the box as well. Thus, 3 can be eliminated from appearing elsewhere in that box.


Happy solving!

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