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A304867 Number of non-isomorphic hypertrees of weight n. 37
1, 1, 1, 1, 2, 2, 5, 6, 13, 20, 41, 70, 144, 266, 545, 1072, 2210, 4491, 9388, 19529, 41286, 87361, 186657, 399927, 862584, 1866461, 4058367, 8852686, 19384258, 42570435, 93783472, 207157172, 458805044, 1018564642, 2266475432, 5053991582, 11292781891, 25280844844 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

A hypertree E is a connected antichain of finite sets satisfying Sum_{e in E} (|e| - 1) = |U(E)| - 1. The weight of a hypertree is the sum of cardinalities of its elements. Weight is generally not the same as number of vertices (see A035053).

From Kevin Ryde, Feb 25 2020: (Start)

a(n), except at n=1, is the number of free trees of n edges (so n+1 vertices) where any two leaves are an even distance apart.  All trees are bipartite graphs and this condition is equivalent to all leaves being in the same bipartite half.  The diameter of a tree is always between two leaves so these trees have even diameter (A000676).

The correspondence between hypertrees and these free trees is described for instance by Bacher (start of section 1.2).  In such a free tree, call a vertex "even" if it is an even distance from a leaf.  The hypertree vertices are these even vertices.  Each hyperedge is the set of vertices surrounding an odd vertex, so hypertree weight is the total number of edges in the free tree.

(End)

LINKS

Andrew Howroyd, Table of n, a(n) for n = 0..500

R. Bacher, On the enumeration of labelled hypertrees and of labelled bipartite trees, arXiv:1102.2708 [math.CO], 2011.

FORMULA

a(n) = Sum_{k=1..floor(n/2)} A318601(n+1-k, k). - Andrew Howroyd, Aug 29 2018

EXAMPLE

Non-isomorphic representatives of the a(6) = 5 hypertrees are the following:

  {{1,2,3,4,5,6}}

  {{1,2},{1,3,4,5}}

  {{1,2,3},{1,4,5}}

  {{1,2},{1,3},{1,4}}

  {{1,2},{1,3},{2,4}}

Non-isomorphic representatives of the a(7) = 6 hypertrees are the following:

  {{1,2,3,4,5,6,7}}

  {{1,2},{1,3,4,5,6}}

  {{1,2,3},{1,4,5,6}}

  {{1,2},{1,3},{1,4,5}}

  {{1,2},{1,3},{2,4,5}}

  {{1,3},{2,4},{1,2,5}}

From Kevin Ryde, Feb 25 2020: (Start)

a(6) = 5 hypertrees of weight 6 and their corresponding free trees of 6 edges (7 vertices).  Each * is an "odd" vertex (odd distance to a leaf).  Each hyperedge is the set of "even" vertices surrounding an odd.

  {1,2,3,4,5,6}       3   2

                       \ /

                      4-*-1      (star 7)

                       / \

                      5   6

  .

  {1,2},{1,3,4,5}               /-3

                      2--*--1--*--4

                                \-5

  .

  {1,2,3},{1,4,5}     2-\       /-4

                         *--1--*

                      3-/       \-5

  .

  {1,2},{1,3},{1,4}    /-*--2

                      1--*--3

                       \-*--4

  .

  {1,2},{2,4},{1,3}   3--*--1--*--2--*--4   (path 7)

(End)

MATHEMATICA

etr[p_] := Module[{b}, b[n_] := b[n] = If[n == 0, 1, Sum[Sum[d*p[d], {d, Divisors[j]}]*b[n - j], {j, 1, n}]/n]; b];

EulerT[v_List] := With[{q = etr[v[[#]]&]}, q /@ Range[Length[v]]];

ser[v_] := Sum[v[[i]] x^(i-1), {i, 1, Length[v]}] + O[x]^Length[v];

c[n_] := Module[{v = {1}}, For[i = 1, i <= Ceiling[n/2], i++, v = Join[{1}, EulerT[Join[{0}, EulerT[v]]]]]; v];

seq[n_] := Module[{u = c[n]}, x*ser[EulerT[u]]*(1 - x*ser[u]) + (1 - x)* ser[u] + x + O[x]^n // CoefficientList[#, x]&];

seq[40] (* Jean-Fran├žois Alcover, Feb 08 2020, after Andrew Howroyd *)

PROG

(PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, 1/n))))-1, -#v)}

c(n)={my(v=[1]); for(i=1, ceil(n/2), v=concat([1], EulerT(concat([0], EulerT(v))))); v}

seq(n)={my(u=c(n)); Vec(x*Ser(EulerT(u))*(1-x*Ser(u)) + (1 - x)*Ser(u) + x + O(x*x^n))} \\ Andrew Howroyd, Aug 29 2018

CROSSREFS

Cf. A007716, A030019, A035053, A048143, A054921, A134955, A134957, A144959, A304911, A304912, A318601.

Sequence in context: A261866 A147766 A034420 * A303931 A028410 A050216

Adjacent sequences:  A304864 A304865 A304866 * A304868 A304869 A304870

KEYWORD

nonn

AUTHOR

Gus Wiseman, May 20 2018

EXTENSIONS

Terms a(10) and beyond from Andrew Howroyd, Aug 29 2018

STATUS

approved

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Last modified April 16 17:01 EDT 2021. Contains 343050 sequences. (Running on oeis4.)