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 A049219 Number of horizontally convex n-ominoes in which the top row has exactly 1 square. 2
 1, 1, 3, 10, 33, 107, 344, 1103, 3535, 11330, 36317, 116415, 373176, 1196243, 3834643, 12292218, 39403561, 126310851, 404898200, 1297929287, 4160602439, 13337099986, 42753000005, 137047709879, 439315949304, 1408257777387 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer Sequences, Vol. 2 (1999), #99.1.8. FORMULA G.f.: x (1-x)^2 (1-2x)/(1-5x+7x^2-4x^3). a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 5. a(n) = A001169(n-1) + A049221(n) for n >= 2. MATHEMATICA a[ n_ ] := a[ n ]=If[ n<5, {1, 1, 3, 10}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ] LinearRecurrence[{5, -7, 4}, {1, 1, 3, 10, 33, 107}, 40] (* Harvey P. Dale, Nov 19 2019 *) CROSSREFS Cf. A001169, A049221. Sequence in context: A053156 A120897 A077825 * A126184 A292397 A060557 Adjacent sequences:  A049216 A049217 A049218 * A049220 A049221 A049222 KEYWORD nonn,easy AUTHOR Dean Hickerson, Aug 10 1999 STATUS approved

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Last modified December 5 15:11 EST 2019. Contains 329753 sequences. (Running on oeis4.)