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R. Parkin, L. J. Lander, and D. R. Parkin, <a href="/A000104/a000104.pdf">Polyomino Enumeration Results</a>, presented at SIAM Fall Meeting, 1967) , and accompanying letter from T. J. Lander (annotated scanned copy) page 9 (incorrect at n=15).
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a(3)=27 24 = 6+7+7+3+1: There fit 6 5-ominoes in a 3x3 square, 7 6-ominoes in a 3x3 square, 7 7-ominoes in a 3x3 square, 3 8-ominoes in a 3x3 square, a 1 9-omino in a 3x3 square. - R. J. Mathar, Jun 07 2020
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Cf. A259088.
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<a href="/index/Pol#polyominoes">Index entries for sequences related to polyominoes</a>
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a(3)=27 = 6+7+7+3+1: There fit 6 5-ominos ominoes in a 3x3 square, 7 6-ominos ominoes in a 3x3 square, 7 7-ominoes in a 3x3 square, 3 8-ominos ominoes in a 3x3 square, a 1 9-omino in a 3x3 square. - R. J. Mathar, Jun 07 2020
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