%I A007576 M2656
%S A007576 1,1,1,3,7,15,35,87,217,547,1417,3735,9911,26513,71581,194681,532481,
%T A007576 1464029,4045117,11225159,31268577,87404465,245101771,689323849,
%U A007576 1943817227,5494808425,15568077235,44200775239,125739619467
%N A007576 Number of solutions to k_1+2*k_2+..+n*k_n=0, where k_i are from {-1,0,
               1}, i=1..n.
%C A007576 Also, number of maximally stable towers of 2 X 2 LEGO blocks.
%D A007576 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A007576 P. J. S. Watson, On "LEGO" towers, J. Rec. Math., 12 (No. 1, 1979-1980), 
               24-27.
%H A007576 T. D. Noe, <a href="b007576.txt">Table of n, a(n) for n=0..100</a>
%H A007576 S. R. Finch, <a href="http://algo.inria.fr/bsolve/">Signum equations 
               and extremal coefficients</a>.
%F A007576 Coefficient of x^(n*(n+1)/2) in Product_{k=1..n} (1+x^k+x^(2*k)).
%e A007576 For n=4 there are 7 solutions: (-1,-1,1,0), (-1,0,-1,1), (-1,1,1,-1), 
               (0,0,0,0), (1,-1,-1,1), (1,0,1,-1), (1,1,-1,0).
%t A007576 f[0] = 1; f[n_] := Coefficient[Expand@ Product[1 + x^k + x^(2k), {k, 
               n}], x^(n(n + 1)/2)]; Table[f@n, {n, 0, 28}] (from Robert G. Wilson 
               v (rgwv(at)rgwv.com), Nov 10 2006)
%Y A007576 Cf. A007575, A063865, A039826.
%Y A007576 Sequence in context: A124696 A081669 A086821 this_sequence A018020 A147106 
               A078161
%Y A007576 Adjacent sequences: A007573 A007574 A007575 this_sequence A007577 A007578 
               A007579
%K A007576 easy,nonn
%O A007576 0,4
%A A007576 Simon Plouffe, Robert G. Wilson v (rgwv(AT)rgwv.com) and Vladeta Jovovic 
               (vladeta(AT)eunet.rs)
%E A007576 More terms from David Wasserman (wasserma(AT)spawar.navy.mil), Mar 29 
               2005
%E A007576 Edited by N. J. A. Sloane (njas(AT)research.att.com), Nov 07 2006. This 
               is a merging of two sequences which thanks to the work of Soren Eilers 
               we now know are identical.