Impossible Sudoku Puzzles - Play Sudoku Online

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Puzzle 8: You Won't Want to Play Sudoku Again, Programming for the Puzzled, Electrical Engineering and Computer Science

Is there a limit to how many people can solve a very difficult sudoku puzzle? - Quora

Sudoku Challenge

Sudoku 10x10 - Easy

Heidi's Infinite Sudoku

Are there incomplete impossible Sudoku puzzles with no immediate conflict in the provided squares? - Quora

Glossary of Sudoku - Wikipedia

Online Sudoku Puzzle, Play Online for Free

SudokuEverywhere - Apps on Google Play

Sudoku - Hard

Sudoku - Hard

The 10 Hardest Logic Puzzles Ever Created