A351616 - OEIS

%I #15 Feb 26 2023 08:42:59

%S 0,0,1,2,12,43,174,657,2571,9911,38633,150437,588544,2306204,9061179,

%T 35665587,140648710,555514177,2197298561,8702439446,34506900216,

%U 136972867380,544237912916,2164377093376,8614630304437

%N Number of free polyominoes with 2n squares, having 180-degree rotational symmetry about a vertex of a square, and no other symmetry.

%C The sequence enumerates a subset of the polyominoes enumerated by A006747.

%e a(3)=1 because of:

%e    OOO

%e   OOO

%Y Cf. A006747, A234008, A351615.

%K nonn,more

%O 1,4

%A _John Mason_, Feb 15 2022

%E a(17)-a(25) from _John Mason_, Feb 26 2023