[seqfan] Re: Help with A028247 Number of T-frame polyominoes with n cells

[mathar]

The T-frame polyominoes describe a subset of stages
of chomp https://en.wikipedia.org/wiki/Chomp
after two bites. It's only a subset because to leave
a T-polyomino the first bite must not remove an
entire set of rows/columns and the second bite must
not do that either and must not nimble at the corner
diagonally opposite to the first bite.