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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.: (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-Fran├žois Alcover, Feb 09 2016 *)

CROSSREFS

Cf. A036654, A036655, A058756, A000084.

Sequence in context: A148458 A148459 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 4 21:51 EST 2016. Contains 278755 sequences.