References

• HoDoKu: https://hodoku.sourceforge.net/en/techniques.php
  This is a long-standing Sudoku techniques site and the main reference source for this site. It explains many foundational classic Sudoku techniques and focuses more on logical solving than on speedsolving. HoDoKu also provides a cross-platform Java GUI application that lets you view the complete solving path for a Sudoku puzzle directly.

• r/sudoku: https://www.reddit.com/r/sudoku/
  A very active Sudoku solving help community on Reddit, with detailed solving techniques also available on its wiki page.

• Sudopedia: https://www.sudopedia.org/wiki/Solving_Technique
  A reference website organized in wiki form, and also an important source for HoDoKu.

• sudoku.coach: https://sudoku.coach/en/learn
  Offers online games, tutorials, and practice for classic Sudoku, supports direct step-by-step solution viewing, and also covers many Sudoku variants.

• The New Sudoku Players' Forum: http://forum.enjoysudoku.com/
  A Sudoku discussion forum.

• SudokuWiki: https://www.sudokuwiki.org/Getting_Started
  Provides a large collection of online digit-placement games, including classic Sudoku and its variants. It also supports direct step-by-step solution viewing and offers extensive documentation resources.