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A000675
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Number of centered 3-valent (or boron, or binary) trees with n nodes.
(Formerly M0977 N0366)
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3
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1, 1, 0, 1, 1, 1, 2, 4, 5, 10, 19, 36, 68, 138, 277, 581, 1218, 2591, 5545, 12026, 26226, 57719, 127685, 284109, 634919, 1425516, 3212890, 7269605, 16504439, 37592604, 85876345, 196717882, 451768247, 1039990913, 2399476030, 5547849750
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listen;
history;
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internal format)
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OFFSET
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0,7
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REFERENCES
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A. Cayley, On the analytical forms called trees, with application to the theory of chemical combinations, Reports British Assoc. Advance. Sci. 45 (1875), 257-305 = Math. Papers, Vol. 9, 427-460 (see p. 451).
R. C. Read, personal communication.
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).
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LINKS
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MATHEMATICA
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n = 50; (* algorithm from Rains and Sloane *)
S2[f_, h_, x_] := f[h, x]^2/2 + f[h, x^2]/2;
S3[f_, h_, x_] := f[h, x]^3/6 + f[h, x] f[h, x^2]/2 + f[h, x^3]/3;
T[-1, z_] := 1; T[h_, z_] := T[h, z] = Table[z^k, {k, 0, n}].Take[CoefficientList[z^(n+1) + 1 + S2[T, h-1, z]z, z], n+1];
Sum[Take[CoefficientList[z^(n+1) + S3[T, h-1, z]z - S3[T, h-2, z]z - (T[h-1, z] - T[h-2, z]) (T[h-1, z]-1), z], n+1], {h, 1, n/2}] + PadRight[{1, 1}, n+1] (* Robert A. Russell, Sep 15 2018 *)
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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STATUS
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approved
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