Improve your skills
For beginners who've just learned the rules, problems are a great way to hone basic skills. Go Problems (free) has 227 problems to start out, and offers packs of problems as in-app purchases for $1 to $3 each. Some of these are standard problems, 500 in a pack, and some are collections of problems, such as the classic Igo Hatsuyo-Ron, or a set of problems created by the great player Go Seigen (Wu Qingyuan). You can go through the problems move by move, see where you've gone wrong, and find the correct moves to solve them.
Igowin Life ($3) also offers problems, many of which are "life and death" problems, where you have to figure out how to save or kill a group of stones. It has more than 2000 problems, and they're displayed at random, and in a random orientation as well, so you may see the same problem many times, to help you learn the moves it's highlighting.
Another way of perfecting your Go skills is by watching and playing through the games of professional Go players. (The game is played professionally, mostly in Japan, China, and Korea.) SmartGo Kifu ($20) lets you do all this and more. (Kifu is the Japanese word for a Go game record.) In addition to providing the game-playing engine that's in SmartGo Player, with boards up to 19-by-19, SmartGo Kifu has more than 2000 problems, and an astonishing 81,000-plus games by Go players from more than a thousand years ago to the present. This game collection is the most impressive resource available, and includes the well-known (among Go players) GoGoD (Go Games on Disk) collection curated by John Fairbairn and T. Mark Hall.
You can view all the games by a specific player--for example, there are more than 2000 games by Cho Chikun, 1300 by Sakata Eio, 1100 by Takemiya Masaki, all 862 games by Go Seigen, and, of course, the 248 known games by Shusaku. You can choose to view a game at random, and play through it. And you can record your own games to analyze them later. You can also import any SGF files of games that you have, and you can even use it as a go board to play games with two players.
Another way to improve your game is by reading Go books. There are books that teach you techniques, others that analyze and comment on games, and others which are collections of problems. One of the most important changes to the Go world was when Go Books (free)--by the developer of the SmartGo apps--was released. This app created a new format for Go books, which includes not only text and game diagrams, but lets you move through the diagrams as well. Diagrams in Go books often contain five, ten, even 20 moves, and it's hard to visualize them on paper. But with books in the Go Books app, you can play through diagrams one move at a time.
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