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. 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. 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.

=See also=
 * Late Move Reductions
 * SEX Algorithm
 * Singular Extensions

=Publications=
 * David Levy (2003). The State of the Art in Man vs. “Machine” Chess. ICGA Journal, Vol. 26, No. 1

=Forum Posts=
 * PV verification heuristic by Peter Jacobi, CCC, July 05, 1998
 * Re: PV verification heuristic by Robert Hyatt, CCC, July 06, 1998
 * @Ed about PV extension by Peter Fendrich, CCC, July 19, 2007

=External Links=
 * Extension Techniques in REBEL (PVS extensions) from Programmer Corner by Ed Schröder

=References=

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