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 A210996 Number of free polyominoes with 2n cells. 5
 1, 1, 5, 35, 369, 4655, 63600, 901971, 13079255, 192622052, 2870671950, 43191857688, 654999700403, 9999088822075, 153511100594603, 2368347037571252, 36695016991712879, 570694242129491412, 8905339105809603405 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS It appears that we can write A216492(n) < A216583(n) < A056785(n) < A056786(n) < a(n) < A210988(n) < A210986(n), if n >= 3. - Omar E. Pol, Sep 16 2012 LINKS Herman Tulleken, Polyominoes 2.2: How they fit together, (2019). Eric Weisstein's World of Mathematics, Polyomino Wikipedia, All 5 free tetrominoes, Illustration of a(2) = 5. Wikipedia, All 35 free hexominoes, Illustration of a(3) = 35. Wikipedia, All 369 free octominoes, Illustration of a(4) = 369. Wikipedia, Polyomino EXAMPLE For n = 1 there is only one free domino. For n = 2 there are 5 free tetrominoes. For n = 3 there are 35 free hexominoes. For n = 4 there are 369 free octominoes (see link section). CROSSREFS Bisection of A000105. Cf. A000988, A056785, A056786, A210988, A210997, A216492, A216583. Sequence in context: A233860 A258902 A125864 * A204290 A059865 A247596 Adjacent sequences:  A210993 A210994 A210995 * A210997 A210998 A210999 KEYWORD nonn AUTHOR Omar E. Pol, Sep 15 2012 STATUS approved

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Last modified October 13 23:38 EDT 2019. Contains 327983 sequences. (Running on oeis4.)