Wow lovely!!! Awesome zugzwang
Extremely hard puzzle. Chessmaster could not solve this!
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Most likely the reason the engines couldn't solve it is because after 6.Kf4, we have 6...Qxa3! 7.Nxa3+ Kb4, and Black is winning despite the loss of his queen (he will soon capture White's b-pawn and proceed to storm his passed pawns down the board). By contrast, 4.Bc5+ at least keeps more material on the board and gives White some drawing chances.
I found this puzzle by Mark Liburkin in one chess book.
I gave chessmaster 1 minute per move and it could not solve it.
Rybka could not solve 5 sec per move but solved after I gave it 10 sec per move.
I needed 4-5 hours of thinking.
White will win easily, after solving this problem.