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A326374
Irregular triangle read by rows where T(n,d) for d|n is the number of (d + 1)-uniform hypertrees spanning n + 1 vertices.
2
1, 3, 1, 16, 1, 125, 15, 1, 1296, 1, 16807, 735, 140, 1, 262144, 1, 4782969, 76545, 1890, 1, 100000000, 112000, 1, 2357947691, 13835745, 33264, 1, 61917364224, 1, 1792160394037, 3859590735, 270670400, 35135100, 720720, 1, 56693912375296, 1, 1946195068359375
OFFSET
1,2
COMMENTS
A hypertree is a connected hypergraph of density -1, where density is the sum of sizes of the edges minus the number of edges minus the number of vertices. A hypergraph is k-uniform if its edges all have size k. The span of a hypertree is the union of its edges.
LINKS
FORMULA
T(n, d) = n!/(d! * (n/d)!) * ((n + 1)/d)^(n/d - 1).
EXAMPLE
Triangle begins:
1
3 1
16 1
125 15 1
1296 1
16807 735 140 1
262144 1
4782969 76545 1890 1
100000000 112000 1
2357947691 13835745 33264 1
The a(4,2) = 15 hypertrees:
{{1,4,5},{2,3,5}}
{{1,4,5},{2,3,4}}
{{1,3,5},{2,4,5}}
{{1,3,5},{2,3,4}}
{{1,3,4},{2,4,5}}
{{1,3,4},{2,3,5}}
{{1,2,5},{3,4,5}}
{{1,2,5},{2,3,4}}
{{1,2,5},{1,3,4}}
{{1,2,4},{3,4,5}}
{{1,2,4},{2,3,5}}
{{1,2,4},{1,3,5}}
{{1,2,3},{3,4,5}}
{{1,2,3},{2,4,5}}
{{1,2,3},{1,4,5}}
MAPLE
T:= n-> seq(n!/(d!*(n/d)!)*((n+1)/d)^(n/d-1), d=numtheory[divisors](n)):
seq(T(n), n=1..20); # Alois P. Heinz, Aug 21 2019
MATHEMATICA
Table[n!/(d!*(n/d)!)*((n+1)/d)^(n/d-1), {n, 10}, {d, Divisors[n]}]
CROSSREFS
Row sums are A320444.
Column d = 1 is A000272.
Sequence in context: A160616 A362847 A168319 * A143565 A143018 A102012
KEYWORD
nonn,look,tabf
AUTHOR
Gus Wiseman, Jul 03 2019
STATUS
approved