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A000220
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Number of asymmetric trees with n nodes (also called identity trees).
(Formerly M2583 N1022)
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14
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1, 0, 0, 0, 0, 0, 1, 1, 3, 6, 15, 29, 67, 139, 310, 667, 1480, 3244, 7241, 16104, 36192, 81435, 184452, 418870, 955860, 2187664, 5025990, 11580130, 26765230, 62027433, 144133676, 335731381, 783859852, 1834104934, 4300433063, 10102854473, 23778351222
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,9
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REFERENCES
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F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Camb. 1998, p. 330.
S. R. Finch, Mathematical Constants, Cambridge, 2003, p. 301 and 562.
F. Harary, Graph Theory. Addison-Wesley, Reading, MA, 1969, p. 232.
F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 66, Eq. (3.3.22).
D. E. Knuth, Fundamental Algorithms, 3d Ed. 1997, pp. 386-88 describes methodology for generating similar sequence rapidly.
R. C. Read and R. J. Wilson, An Atlas of Graphs, Oxford, 1998.
A. J. Schwenk, 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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Peter Steinbach, Field Guide to Simple Graphs, Volume 1, Part 17 (For Volumes 1, 2, 3, 4 of this book see A000088, A008406, A000055, A000664, respectively.)
Peter Steinbach, Field Guide to Simple Graphs, Volume 3, Part 12 (For Volumes 1, 2, 3, 4 of this book see A000088, A008406, A000055, A000664, respectively.)
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FORMULA
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G.f.: A(x)-A^2(x)/2-A(x^2)/2, where A(x) is g.f. for A004111.
a(n) ~ c * d^n / n^(5/2), where d = A246169 = 2.51754035263200389079535..., c = 0.29938828746578432274375484519722721162... . - Vaclav Kotesovec, Aug 25 2014
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MAPLE
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with(numtheory):
b:= proc(n) option remember; `if`(n<2, n, add(b(n-k)*add(
b(d)*d*(-1)^(k/d+1), d=divisors(k)), k=1..n-1)/(n-1))
end:
a:= n-> b(n)-(add(b(j)*b(n-j), j=0..n)+
`if`(irem(n, 2)=0, b(n/2), 0))/2:
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MATHEMATICA
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s[ n_, k_ ] := s[ n, k ]=a[ n+1-k ]+If[ n<2k, 0, -s[ n-k, k ] ]; a[ 1 ]=1; a[ n_ ] := a[ n ]=Sum[ a[ i ]s[ n-1, i ]i, {i, 1, n-1} ]/(n-1); Table[ a[ i ]-Sum[ a[ j ]a[ i-j ], {j, 1, i/2} ]+If[ OddQ[ i ], 0, a[ i/2 ](a[ i/2 ]-1)/2 ], {i, 1, 50} ] (* Robert A. Russell *)
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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STATUS
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approved
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