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 A007575 Number of stable towers of 2 X 2 LEGO blocks. (Formerly M2675) 2
 1, 3, 7, 19, 53, 149, 419, 1191, 3403, 9755, 28077, 81097, 234861, 681697, 1982723, 5777375, 16861521, 49281525, 144222987, 422566835, 1239423303, 3638872529, 10693065215, 31448140529, 92558787745, 272612601065, 803448576111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 REFERENCES Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 65. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). P. J. S. Watson, On "LEGO" towers, J. Rec. Math., 12 (No. 1, 1979-1980), 24-27. LINKS Ray Chandler, Table of n, a(n) for n = 0..2098 (terms < 10^1000) P. J. S. Watson, On "LEGO" towers, J. Rec. Math., 12 (No. 1, 1979-1980), 24-27. (Annotated scanned copy) FORMULA a(n) ~ 3^(n+1) / sqrt(Pi*n). - Vaclav Kotesovec, Jul 11 2018 MAPLE seq(sum(coeff(product(1+x^k+x^(2*k), k=1..n), x, l), l=n*(n+1)/2-n..n*(n+1)/2+n), n=0..20); # Søren Eilers CROSSREFS Cf. A007576. Sequence in context: A090378 A007180 A059506 * A026299 A183117 A183124 Adjacent sequences:  A007572 A007573 A007574 * A007576 A007577 A007578 KEYWORD nonn AUTHOR STATUS approved

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Last modified January 23 01:30 EST 2020. Contains 331166 sequences. (Running on oeis4.)