Extreme difficulty (maybe unsolvable?) killer sudoku : r/sudoku

What are the points to keep in mind to solve difficult Sudoku puzzles? - Quora

A-Z Sudoku, PDF, Vertex (Graph Theory)

Sudoku Exchange

How hard is this Killer Sudoku? : r/sudoku

Unsolvable Sudoku Puzzles

Sudoku Solver in Python - Lior Sinai

Help! I found this killer cage that seems impossible. It has no givens and follows standard killer cage rules. I haven't even put a single pencil mark. : r/sudoku

Are some Sudoku games impossible to solve? - Quora

8x8 Irregular Killer Sudoku : r/sudoku

Sudoku smashing solution. Exhaustive Sudoku solving bot tutorial…, by Jesko Rehberg

Sudoku Assistant -- Solving Techniques

Sudoku Assistant -- Solving Techniques

r/Sudoku: a place to learn it and enjoy it