Number of Free Polyominoes

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Sequence

The sequence of the number $P_n$ of distinct free polyominoes for each $n$ begins:

$1, 1, 2, 5, 12, 35, 108, 369, 1285, \ldots$

This sequence is A000105 in the On-Line Encyclopedia of Integer Sequences (N. J. A. Sloane (Ed.), 2008).