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A001383
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Number of n-node rooted trees of height at most 3.
(Formerly M1107 N0422)
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19
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1, 1, 1, 2, 4, 8, 15, 29, 53, 98, 177, 319, 565, 1001, 1749, 3047, 5264, 9054, 15467, 26320, 44532, 75054, 125904, 210413, 350215, 580901, 960035, 1581534, 2596913, 4251486, 6939635, 11296231, 18337815, 29692431, 47956995, 77271074, 124212966
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OFFSET
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0,4
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COMMENTS
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a(n+1) is also the number of n-vertex graphs that do not contain a P_4, C_4, or K_4 as induced subgraph (K_4-free trivially perfect graphs, cf. A123467). - Falk Hüffner, Jan 10 2016
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REFERENCES
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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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FORMULA
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G.f.: S[ 3 ] := x*Product (1 - x^k)^(-p(k-1)), where p(k) = number of partitions of k.
G.f.: 1 + x*exp( Sum_{n>=1} x^n/n * Product_{k>=1} 1/(1 - x^(n*k)) ). - Paul D. Hanna, Nov 01 2012
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MAPLE
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s[ 2 ] := x/product('1-x^i', 'i'=1..30); # G.f. for trees of ht <=2, A000041
for k from 3 to 12 do # gets g.f. for trees of ht <= 3, 4, 5, ...
s[ k ] := series(x/product('(1-x^i)^coeff(s[ k-1 ], x, i)', 'i'=1..30), x, 31); od:
# For Maple program see link in A000235.
with(numtheory): etr:= proc(p) local b; b:=proc(n) option remember; local d, j; if n=0 then 1 else add(add(d*p(d), d=divisors(j)) *b(n-j), j=1..n)/n fi end end: A000041:= etr(n-> 1): a:= n->`if`(n=0, 1, etr(k-> A000041(k-1))(n-1)): seq(a(n), n=0..40); # Alois P. Heinz, Sep 08 2008
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MATHEMATICA
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m = 36; CoefficientList[ Series[x*Product[(1 - x^k)^(-PartitionsP[k - 1]), {k, 1, m}], {x, 0, m}], x] // Rest // Prepend[#, 1] & (* Jean-François Alcover, Jul 05 2011, after g.f. *)
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PROG
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(PARI) {a(n)=polcoeff(1+x*exp(sum(m=1, n, x^m/m/prod(k=1, n\m+1, 1-x^(m*k)+x*O(x^n)))), n)} \\ Paul D. Hanna, Nov 01 2012
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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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