Sudoku, or number placement, puzzles are sweeping the country. Even though the game involves numbers, it is not mathematical. And people who feel they don't have a good vocabulary will choose them in preference to crossword puzzles. Soduku is nothing more than logic.

Howard Gams is given credit for inventing the puzzle in 1979. His puzzles never caught on in this country but became an instant success in Japan. Now it is carried by more newspapers than crossword puzzles.

Soduku is a nine-by-nine cubical grid. When properly completed, each of the rows and files will contain all the numbers between 1 and 9 as will each of the 3 X 3 cubes comprising the whole. The puzzle will appear as a completed grid with certain numbers missing. When read across and down, each of the three contiguous inner cubes will have a different digit in a different position. For our purposes we can designate these rows as ranks and files. Ranks run horizontally across the puzzle and files, vertically. If, for example, the number one appears in rank 1 in cube one and rank 3 in cube 2, the third one must necessarily appear in the second rank in cube 3. This would seem to imply that it could appear in any of three positions in the second rank, but most often, there will already be a number in one position, leaving a choice of two possibilities. Let's assume you have a choice of two positions.

The next step is to check the files. If by some chance a one appears in one of the files of your choices, you now know the numeral must be placed in the other. This simple cross check should be your first step in solving the puzzle. Anytime you find a HARD number, one you have logically determined cannot be another, I recommend you write it in with a Pen. This will save unnecessary erasures when you make an inevitable mistake.

Your second step should be to scan each of the smaller cubes for their missing numbers. Usually you will be able to place one or two numbers by the rank-file cross-checking. After you have done that, read across each of the ranks and files to determine the missing numbers. Let's say that you are missing a number one in the top rank and you have missing numbers in all of the smaller cubes. Let's say that the middle cube has a number one as one of the given numbers. That means that the number one you need to place must be located in the left or right cube. Instead of 9 potential placements, you now have only 6.

After you have determined all the missing numbers, you will find that you have many with two or three possibilities. Hopefully you will be able to write these possibilities in the squares. The best you can hope for is to have two contiguous squares with only two possibilities. In the top rank, squares one and two are either a six or an eight, but you can't tell which. You still have learned a great deal of information because it means that those two numbers can't appear in any of the other empty squares across that rank. As I suggested before, every time you find a "hard number," one that cannot be placed anywhere else, fill it in with pen.

Another way to come up with information is to use the "what if" method of placing numbers. Let's say that you have a six in the second rank of the small cube in the upper left corner. Even if you have no other information where the other two 6s are located, you you know that the other 6s must be in first and third ranks. By placing it in the first rank, you will then have to locate the last placement in the third rank of the remaining cube. By testing the placements you will often run into a contradiction in one of the ranks or files. If you don't shrug your shoulders and try placing another number and see what happens.

Kerep at it. Sooner or later you will have filled in most of the numbers. On more than one occasion I have run into a contradicition on the very last number. There's nothing to do about it but back up and start again. Patience is the greatest skill required to solve thse puzzles.

Sooner or later, you, your pencil the paper or the puzzle will call it quits. Let's hope it's the puzzle.