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 A000137 Series-parallel numbers. (Formerly M1625 N0636) 2
 1, 2, 6, 18, 58, 186, 614, 2034, 6818, 22970, 77858, 264970, 905294, 3102434, 10661370, 36722642, 126752218, 438294018, 1518032598, 5265341314, 18286911130, 63586988434, 221342104842, 771235606050, 2689688538646, 9388096331642 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 142. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Sean A. Irvine, Table of n, a(n) for n = 1..1000 FORMULA G.f.: x(1+S)/(1-S), where S = g.f. for A000084. MATHEMATICA 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 *) CROSSREFS Cf. A036654, A036655, A058756, A000084. Sequence in context: A307755 A304200 A081057 * A151282 A193777 A157004 Adjacent sequences: A000134 A000135 A000136 * A000138 A000139 A000140 KEYWORD nonn,easy AUTHOR N. J. A. Sloane STATUS approved

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Last modified December 1 23:26 EST 2023. Contains 367503 sequences. (Running on oeis4.)