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A007099
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Number of labeled trivalent (or cubic) 2-connected graphs with 2n nodes.
(Formerly M5344)
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2
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0, 1, 70, 19320, 11052720, 11408720400, 19285018552800, 49792044478176000, 186348919238786304000, 970566620767088881536000, 6808941648018137282054400000, 62642603299257346706851910400000
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,3
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REFERENCES
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R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, Department of Mathematics, Univ. London, 1958.
R. W. Robinson, personal communication.
R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1976.
R. W. Robinson, Computer print-out, no date. Gives first 29 terms.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = (2*n)! * (s(n) - 2*s(n-1)) / (3*n*2^n) where s(1)=0, s(2)=1, and s(n) = 3*n*s(n-1) + 2*s(n-2) + (3*n-1) * Sum_{i=2..n-3} s(i) * s(n-1-i). - Sean A. Irvine, Oct 11 2017
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MAPLE
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s := proc(n)
option remember;
if n = 1 then
0;
elif n = 2 then
1;
else
3*n*procname(n-1)+2*procname(n-2)+(3*n-1)*add(procname(i)*procname(n-1-i), i=2..n-3) ;
end if;
end proc:
if n = 1 then
0;
elif n = 2 then
1;
else
(2*n)!/3/n/2^n*(s(n)-2*s(n-1)) ;
end if;
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MATHEMATICA
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s[n_] := s[n] = If[n <= 2, n - 1, 3 n s[n - 1] + 2 s[n - 2] + (3 n - 1) Sum[s[i] s[n - 1 - i], {i, 2, n - 3}]]; Array[Floor[(2 #)!*(s[#] - 2 s[# - 1])/(3 # 2^#)] &, 12] (* Michael De Vlieger, Oct 11 2017 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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