Sudoku - Wikipedia

File:Sudoku Puzzle (a puzzle with total symmetry).png - Wikipedia

Sudoku Gridmaster for Nintendo DS - Sales, Wiki, Release Dates, Review, Cheats, Walkthrough

Hardest puzzle I've ever seen : r/sudoku

File:Sudoku Puzzle (a puzzle with total symmetry) trimmed.png - Wikipedia

Kakuro - Wikipedia

Glossary of Sudoku - Wikipedia

Sudoku graph - Wikipedia

Sudoku, PDF, Popular Scholarship

According to wikipedia, this is currently the least known clues killer sudoku (no killer sums, just eight < > = signs). I solved it and it's amazing : r/sudoku