Okay, but... – Sudoku - Brain Puzzle Games Review

Expert puzzle solutions make use of X-Y chains, which requires you to try a solution until you reach a contradiction. This is essentially guessing, and the purist puzzle master never prepares a puzzle requiring you to guess. Superior puzzles are available through the sudoku.com app. Their expert puzzles are the most brutal I’ve seen that are always solvable through deductive analysis.
Review by Mikeyz3 on Sudoku - Brain Puzzle Games.