Cool Chess Puzzle #12 - A.A. Troitsky
This puzzle by A.A. Troitsky from 1895, has multiple key concepts one must discover in order to solve correctly. Interestingly enough, when I tried to have Stockfish 6 solve this endgame study, it couldn't. Only deeper into the puzzle did I realize why. This might be a fun puzzle to test out with your chess engine or chess computer. This endgame study was published in "Nowoje Wremja".