SUDOKU RULES
HOW TO PLAY
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 19 exactly once. The
solution to each puzzle is unique.
USING THE ERASERFREE FORMAT
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.
TIPS ON SOLVING 
(1)
BoxStripping: Look at any set of three
sidebyside 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. Preexisting 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
crosschecking  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)
BoxFlushing: EraserFree 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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