The rules: insert the digits 1-9 just once in each a) row, b) column, c) bold outlined area, d) gray area and e) both long diagonals. See Ritmeester's author page for his 18 other books. Peter Ritmeester is the inventor of the Hyper Sudoku , where four gray regions also contain 1-9. For more than a decade he has published these Hypers in The International New York Times and many other newspapers worldwide. The four gray regions provide for some nice...
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