A123772 Number of ways, counted up to symmetry, to build a contiguous building with n LEGO blocks of size 1 X 3 which is flat, i.e., with all blocks in parallel position.
1, 3, 17, 107, 761, 5607, 42730, 331851, 2618024, 20895964, 168398683, 1367838390, 11184711448, 91976811135
Offset: 1