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 A320314 a(n) is the number of symmetric domino towers with n bricks. 3
 1, 1, 3, 3, 7, 9, 19, 25, 53, 71, 149, 203, 423, 583, 1209, 1681, 3473, 4863, 10017, 14107, 28987, 41019, 84113, 119513, 244645, 348829, 712987, 1019731, 2081547, 2985097, 6086375, 8749185, 17820657, 25671983, 52241825, 75402907, 153316715, 221673707, 450393329, 652234089 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A domino tower is a stack of bricks, where (1) each row is offset from the preceding row by half of a brick, (2) the bottom row is contiguous, and (3) each brick is supported from below by at least half of a brick. The number of (not necessarily symmetric) domino towers with n blocks is given by 3^(n-1). a(n) is odd for all n. The not necessarily symmetric case is described in the Miklos Bona reference. Similar considerations lead to a decomposition of symmetric towers into half pyramids which are enumerated by the Motzkin numbers. - Andrew Howroyd, Mar 12 2021 REFERENCES Miklos Bona, editor, Handbook of Enumerative Combinatorics, CRC Press, 2015, pages 25-27. LINKS Andrew Howroyd, Table of n, a(n) for n = 1..1000 Peter Kagey, Symmetric Brick Stacking, Math Stack Exchange. FORMULA G.f.: (x + 2*x^3*M(x^2) + x^2*M(x^2))/((1-x^3*M(x^2))*(1-x^2*M(x^2))) where M(x) is the g.f. of A001006. - Andrew Howroyd, Mar 12 2021 EXAMPLE For n = 4, the a(4) = 3 symmetric stacks are     +-------+     |       | +---+---+---+---+ |       |       | +---+---+---+---+,     |       |     +-------+ +-------+       +-------+ |       |       |       | +---+---+---+---+---+---+, and     |       |       |     +-------+-------+ +-------+-------+-------+-------+ |       |       |       |       | +-------+-------+-------+-------+. PROG (PARI) seq(n)={my(h=(1 - x^2 - sqrt(1-2*x^2-3*x^4 + O(x^3*x^n)))/(2*x^2)); Vec((x + 2*x*h + h)/((1-x*h)*(1-h)))} \\ Andrew Howroyd, Mar 12 2021 CROSSREFS Cf. A000244, A001006, A168368, A264746. Sequence in context: A048240 A122012 A185306 * A056295 A117525 A075149 Adjacent sequences:  A320311 A320312 A320313 * A320315 A320316 A320317 KEYWORD nonn AUTHOR Peter Kagey, Oct 10 2018 EXTENSIONS a(20)-a(40) from Andrew Howroyd, Oct 25 2018 STATUS approved

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Last modified June 29 17:41 EDT 2022. Contains 354913 sequences. (Running on oeis4.)