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 A049220 Number of horizontally convex n-ominoes in which the top row has at least 2 squares and the rightmost square in the top row is above the leftmost square in the second row. 3
 0, 0, 1, 3, 9, 28, 89, 285, 914, 2931, 9397, 30124, 96565, 309545, 992266, 3180775, 10196193, 32684604, 104772769, 335856389, 1076610978, 3451151243, 11062904925, 35462909836, 113678819677, 364405349233, 1168126647770 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 LINKS Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer Sequences, Vol. 2 (1999), #99.1.8. FORMULA G.f.: x^3 (1-x)^2/(1-5x+7x^2-4x^3). a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6. a(n) = a(n-1) + A001169(n-2) for n >= 3. MATHEMATICA a[ n_ ] := a[ n ]=If[ n<6, {0, 0, 1, 3, 9}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ] CROSSREFS Cf. A001169. Sequence in context: A170953 A333504 A199104 * A094790 A007822 A094164 Adjacent sequences:  A049217 A049218 A049219 * A049221 A049222 A049223 KEYWORD nonn,easy AUTHOR Dean Hickerson, Aug 10 1999 STATUS approved

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Last modified July 23 19:17 EDT 2021. Contains 346259 sequences. (Running on oeis4.)