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A005218 Number of unlabeled reduced unit interval graphs on n nodes.
(Formerly M2369)
1
0, 0, 1, 1, 3, 4, 11, 21, 55, 124, 327, 815, 2177, 5712, 15465, 41727, 114291, 313504, 866963, 2404251, 6701321, 18733340, 52557441, 147849031, 417080105, 1179355476, 3342487033, 9492629497, 27011665839, 77000574224 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

REFERENCES

R. W. Robinson, personal communication.

R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1980.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

R. W. Robinson, Table of n, a(n) for n = 1..190

Phil Hanlon, Counting interval graphs, Trans. Amer. Math. Soc. 272 (1982), no. 2, 383-426.

FORMULA

G.f.: -z+(1/4)(1+2z-z^2)/sqrt[(1+z^2)(1-3z^2)]-(1/4)sqrt[(1-3z)/(1+z)]. - Emeric Deutsch, Nov 19 2004

MAPLE

G:=-z+(1+2*z-z^2)/4/sqrt((1+z^2)*(1-3*z^2))-sqrt((1-3*z)/(1+z))/4: Gser:=series(G, z=0, 30): seq(coeff(Gser, z^n), n=1..28); # Emeric Deutsch, Nov 19 2004

CROSSREFS

Sequence in context: A001642 A001643 A247171 * A219514 A131481 A001072

Adjacent sequences:  A005215 A005216 A005217 * A005219 A005220 A005221

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms from Emeric Deutsch, Nov 19 2004

STATUS

approved

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Last modified September 17 10:52 EDT 2019. Contains 327129 sequences. (Running on oeis4.)