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A209080
Number of chess diagrams that can be obtained in exactly one way in n plies and cannot be obtained in fewer plies. This is also the number of dual-free shortest proof games in n plies.
2
1, 20, 400, 1702, 8659, 49401, 287740, 1934794, 11569093, 65443733, 360231372, 1872156836
OFFSET
0,2
COMMENTS
Chess diagrams are chess positions without regard to whether castling or en passant capturing is possible. They are counted in A019319. A shortest proof game (SPG) is a classic type of chess problem. Most published SPGs are dual-free (have only one solution), which is why we count diagrams that are obtained in exactly one way in the minimum number of plies.
CROSSREFS
KEYWORD
nonn,hard,fini,more
AUTHOR
François Labelle, Mar 04 2012
EXTENSIONS
a(11) from François Labelle, Feb 27 2017
STATUS
approved