Sudoku is arguably the most popular logic puzzle ever created. Its name is an abbreviation of a Japanese phrase, which translated means "the digits must occur only once". This completely describes the rules of the puzzle - in every row, column and outlined 3x3 square all numbers from 1 to 9 should appear without repetitions.
The Sudokus below (sorted by difficulty) are shared by KrazyDad, a website with thousands of various types puzzle problems.
Remark: All materials are copyrighted, for personal use only. If you want to use any of them for a book, app, website or periodical, please contact the authors first.