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 A056780 Rectangular free polyominoes: number of n-celled polyominoes when the cell is a rectangle. 7
 1, 2, 3, 9, 21, 68, 208, 730, 2542, 9287, 34053, 127112, 476849, 1803636, 6851960, 26157362, 100211446, 385239872, 1485232325, 5741327939, 22246061118, 86383655207, 336093789246, 1309999171971, 5114453234510, 19998176771431, 78306018629550, 307022197845116 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS R. J. Mathar, Illustration of shapes up to 7-ominoes Ed Pegg, Jr., Illustrations of polyforms M. Vicher, Polyforms M. Vicher, The 21 5-celled rectangular polyominoes M. Vicher, The 21 5-celled rectangular polyominoes Eric Weisstein's World of Mathematics, Polyrect [From Eric W. Weisstein, Apr 24 2009] Wikipedia, Polyomino FORMULA a(n) = 2*A006749(n) + 2*A006746(n) + A006748(n) + 2*A006747(n) + 2*A056877(n) + A056878(n) + A144553(n) + A142886(n). - Andrew Howroyd, Dec 04 2018 MATHEMATICA A[s_] := With[{s6 = StringPadLeft[ToString[s], 6, "0"]}, Cases[Import[ "https://oeis.org/A" <> s6 <> "/b" <> s6 <> ".txt", "Table"], {_, _} ][[All, 2]]]; A006749 = A@006749; A006746 = A@006746; A006748 = A@006748; A006747 = A@006747; A056877 = A@056877; A056878 = A@056878; A144553 = A@144553; A142886 = A@142886; a[n_] := 2*A006749[[n]] + 2*A006746[[n]] + A006748[[n]] + 2*A006747[[n]] + 2*A056877[[n]] + A056878[[n]] + A144553[[n]] + A142886[[n + 1]]; Array[a, 28] (* Jean-François Alcover, Mar 26 2020 *) CROSSREFS Cf. A000105 (cell is square), A151522 (1-sided), A001168 (fixed). Sequence in context: A099607 A077550 A301809 * A324614 A141505 A111360 Adjacent sequences:  A056777 A056778 A056779 * A056781 A056782 A056783 KEYWORD nice,nonn,hard AUTHOR James A. Sellers, Aug 28 2000 EXTENSIONS Edited by N. J. A. Sloane, Apr 25, 2001 Two more terms from Ed Pegg Jr, May 13 2009 a(13)-a(18) from Joseph Myers, Nov 15 2010 a(19)-a(28) from Andrew Howroyd, Dec 04 2018 STATUS approved

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Last modified December 1 14:10 EST 2021. Contains 349430 sequences. (Running on oeis4.)