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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.
0
1, 3, 17, 107, 761, 5607, 42730, 331851, 2618024, 20895964, 168398683, 1367838390, 11184711448, 91976811135
OFFSET
1,2
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Søren Eilers, Oct 29 2006
STATUS
approved