A free program for Android, by Chess King.

The course is an advanced one, which contains material from the game of chess. The program gives tasks to solve and helps to solve them if you get stuck. It will give you hints, explanations, and show you even striking refutation of the mistakes you might make.

The course is interactive. You can not only read the text of the lessons, but also make moves on the board and work out unclear moves on the board.

The program includes a theory section, which explains the methods of the game in a certain stage of the game, based on actual examples.