Depth Reduction R

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R is a common name for a constant or variable signifying depth reduction used in the Null Move Pruning [1], Null Move Reductions and independently in Multi-Cut [2]. In many null move implementations it is either 2 or 3, the exact formula being depth - R - 1.

Variable Reduction

When R becomes a variable, then we say that a program uses adaptive null move pruning, first described by Ernst A. Heinz [3], using R=3 when normal search depth exceeds 6 plies and R=2 otherwise, while more recently most engines account increased search depth with greater reductions [4].

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  1. Chrilly Donninger. (1993). Null Move and Deep Search: Selective-Search Heuristics for Obtuse Chess Programs. ICCA Journal, Vol. 16, No. 3
  2. Yngvi Björnsson, Tony Marsland (2001). Multi-cut Alpha-Beta Pruning in Game Tree Search. Theoretical Computer Science, Vol. 252
  3. Ernst A. Heinz. (1999). Adaptive null-move pruning. ICCA Journal, Vol. 22, No 3
  4. Rethinking r in null move by Michael Sherwin, CCC, August 18, 2017

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