Sudoku :  four levels of difficulty 

 - 6 volumes :  Illustrations ;  21 cm 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Mathematical puzzles<br/>Mathematical recreations<br/>Number puzzles
<br/><br/><label>Index Terms--Genre/Form: </label>--puzzles
<br/><br/><label>Dewey Class. No.: </label>793.73