PV Extensions

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PV Extensions, (PVS Extensions) a technique to extend along the principal variation (PV) to achieve higher search depths in the critical part of the tree. In Rebel, Ed Schröder incremented a counter each time two consecutive PV-moves were retrieved from the transposition table, to extend by one ply each time the counter is divisible by four ¹. Influenced by observing the principal variations of Junior during the Kasparov versus Deep Junior 2003 match, David Levy proposed multiple extensions to treat the often early stable part of a PV as a single ply in 2003 ². To determine the length of the PV, Levy expresses a threshold as a percentage of the total number of nodes searched with the common PV during the previous iteration of the iterative deepening.