Well, you can try:
https://itunes.apple.com/us/app/smallfish-chess-for-stockfish/id675049147?mt=8
This is a free app with a strong engine (Stockfish). Someone else might able to suggest you other alternatives. Also try the Stockfish iOS app.
Free iOS chess AI?
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I've just got into the Apple iOS. I'm looking for a free (ie. no ads and no limitation) and strong computer chess AI for me to train with. Any suggestion? Can we make a list of it?