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%I M1625 N0636 #31 Aug 08 2017 08:44:24
%S 1,2,6,18,58,186,614,2034,6818,22970,77858,264970,905294,3102434,
%T 10661370,36722642,126752218,438294018,1518032598,5265341314,
%U 18286911130,63586988434,221342104842,771235606050,2689688538646,9388096331642
%N Series-parallel numbers.
%D J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 142.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Sean A. Irvine, <a href="/A000137/b000137.txt">Table of n, a(n) for n = 1..1000</a>
%F G.f.: x(1+S)/(1-S), where S = g.f. for A000084.
%t n = 30; s = 1/(1 - x) + O[x]^(n + 1); Do[s = s/(1 - x^k)^Coefficient[s, x^k] + O[x]^(n + 1), {k, 2, n}] ; S = s - 1; CoefficientList[(1 + S)/(1 - S) + O[x]^n, x] (* _Jean-François Alcover_, Feb 09 2016 *)
%Y Cf. A036654, A036655, A058756, A000084.
%K nonn,easy
%O 1,2
%A _N. J. A. Sloane_
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