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a chess engine by [[Robert Houdart]], which appeared in 2010 as closed source engine, free for non-commercial use. In January 2011, Houdini '''1.5''' was leading [[Ingo Bauer|Ingo Bauer's]] [[IPON]] [[Engine Rating Lists|rating list]] <ref>[http://www.inwoba.de/News2009_2011.7z IPON-Rating-List Archive 2009/2010/2011] by [[Ingo Bauer]]</ref>. Houdini '''2''', released in September 2011, is commercial and bundled with a number of [[ChessOK]] products, such as [[Aquarium]], [[Chess Assistant]] <ref>[http://chessok.com/?p=25808 Houdini 2 Aquarium products family has been released!], [[ChessOK]], October 07, 2011</ref>, and since December 2011, [[Chess King]] <ref>[http://chessok.com/?p=26441 Chess King Released], [[ChessOK]], December 14, 2011</ref>. Houdini 1.5a remains available as a free download <ref>[http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=421920&t=40245 Re: Houdini 2 is released] by [[Robert Houdart]], [[CCC]], September 02, 2011</ref>. Houdini '''3''', released in October 2012, has further improved by more than 50 Elo points <ref>[http://www.inwoba.de/index.html IPON-Rating-List, 2012.10.17] by [[Ingo Bauer]]</ref> . According to its author, Houdini 3 applies a accelerated [[Principal Variation Search|principal variation search]], also dubbed "Smart" [[Fail-High]]. If a different move becomes [[Best Move|best]] at very high [[Depth|search depths]] at the [[Root|root]], re-searches to determine its [[Exact Score|exact score]] and the new [[Principal Variation|principal variation]] are done faster due to a reduced search depth, presumably in conjunction with [[Extensions|extensions]] along the PV <ref>[http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=488422&t=45624 Re: Houdini 3 reducing the depth feature] by [[Robert Houdart]], [[CCC]], October 19, 2012</ref>. Since November 2012, Houdini 3 is available as [[ChessBase]] engine <ref>[http://chessbase-shop.com/en/products/houdini_3_standard_multiprocessor_version Houdini 3 Standard multiprocessor version], [[ChessBase|ChessBase Shop]]</ref> . Houdini '''4''', released in November 2013, was about 50 Elo stronger than its predecessor, and features 6-men [[Syzygy Bases|Syzygy bases]].