"The square root is only based on the assumption of perfect alpha-beta pruning.
So it depends on the efficiency of the engine: 0.67 for Chinook of Schaeffer for Checkers and close to 0.5 for modern Chess engines, which have evolved more over years."
no justification given for the 0.5. in fact, most modern chess engines are actually far less efficient.
in addition, if you actually read the solution paper to checkers, you would have noticed how schaeffer points out that an alpha beta search does not prevent double counting positions.
@12336
"the square root formula is based on assumptions of constant branching factor, games of equal length and no transpositions"
++ No. The square root is only based on the assumption of perfect alpha-beta pruning.
So it depends on the efficiency of the engine: 0.67 for Chinook of Schaeffer for Checkers and close to 0.5 for modern Chess engines, which have evolved more over years.
Variable branching factor, games of unequal length and transpositions do not affect that.
To cope with transpositions it is enough to consider an equivalent branching factor of non-transposing moves. That is also how an engine works: if a branch reaches a transposition, then that branch is not calculated, but looked up in the transposition table.
"Transpositions are ubiquitous in chess analysis." ++ Yes.
As 10^38 = 3^80 a game of average 40 moves has only 3 non transposing choices per ply.
"all positions with promotions or multiple promotions"
++ No. Underpromotions are rare. Promotions to pieces not previously captured are rare. Underpromotions to pieces not previously captured do not occur in games with optimal play from both sides and can be ignored indeed.
"states can be reached in an ENORMOUS number of ways" ++ Yes, that is why positions, not games count. That is why a way to handle transpositions is necessary i.e. a transposition table.