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 A084481 Number of fault-free tilings of a 4 X 2n rectangle with L tetrominoes. 5
 2, 6, 10, 18, 38, 84, 186, 410, 904, 1994, 4398, 9700, 21394, 47186, 104072, 229538, 506262, 1116596, 2462730, 5431722, 11980040, 26422810, 58277342, 128534724, 283492258, 625261858, 1379058440, 3041609138, 6708480134, 14796018708, 32633646554, 71975773242 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Fault-free tilings are those where the only straight interface is at the left and right end. Thus a(n) <= A084480(n). LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (2,0,1). FORMULA G.f.: 2*z*(1+z)^2*(1-z-z^3) / (1-2*z-z^3). a(n) = 2*a(n-1) + a(n-3) for n>6. - Colin Barker, Mar 28 2017 PROG (PARI) Vec(2*x*(1 + x)^2*(1 - x - x^3) / (1 - 2*x - x^3) + O(x^30)) \\ Colin Barker, Mar 28 2017 CROSSREFS Cf. A084478, A084479, A084480, A084477. Sequence in context: A174316 A095357 A095358 * A006553 A054273 A127567 Adjacent sequences:  A084478 A084479 A084480 * A084482 A084483 A084484 KEYWORD nonn,easy AUTHOR Ralf Stephan, May 27 2003 STATUS approved

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Last modified October 23 22:21 EDT 2019. Contains 328373 sequences. (Running on oeis4.)