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A331933
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Number of semi-lone-child-avoiding rooted trees with at most one distinct non-leaf branch directly under any vertex.
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10
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1, 1, 1, 2, 4, 6, 12, 18, 33, 52, 90, 142, 242, 384, 639, 1028, 1688, 2716, 4445, 7161, 11665, 18839, 30595, 49434, 80199, 129637, 210079, 339750, 550228, 889978, 1440909, 2330887, 3772845, 6103823, 9878357, 15982196, 25863454, 41845650, 67713550, 109559443
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OFFSET
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1,4
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COMMENTS
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A rooted tree is semi-lone-child-avoiding if there are no vertices with exactly one child unless the child is an endpoint/leaf.
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LINKS
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FORMULA
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a(n) = 1 + Sum_{i=2..n-2} floor((n-1)/i)*a(i). - Andrew Howroyd, Feb 09 2020
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EXAMPLE
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The a(1) = 1 through a(8) = 18 trees:
o (o) (oo) (ooo) (oooo) (ooooo) (oooooo)
(o(o)) (o(oo)) (o(ooo)) (o(oooo))
(oo(o)) (oo(oo)) (oo(ooo))
((o)(o)) (ooo(o)) (ooo(oo))
(o(o)(o)) (oooo(o))
(o(o(o))) ((oo)(oo))
(o(o(oo)))
(o(oo(o)))
(oo(o)(o))
(oo(o(o)))
((o)(o)(o))
(o((o)(o)))
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MATHEMATICA
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sseo[n_]:=Switch[n, 1, {{}}, 2, {{{}}}, _, Join@@Function[c, Select[Union[Sort/@Tuples[sseo/@c]], Length[Union[DeleteCases[#, {}]]]<=1&]]/@Rest[IntegerPartitions[n-1]]];
Table[Length[sseo[n]], {n, 10}]
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PROG
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(PARI) seq(n)={my(v=vector(n)); for(n=1, n, v[n] = 1 + sum(i=2, n-2, ((n-1)\i)*v[i])); v} \\ Andrew Howroyd, Feb 09 2020
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CROSSREFS
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Not requiring lone-child-avoidance gives A320222.
Matula-Goebel numbers of these trees are A331936.
Numbers S with at most one distinct prime index in S are A331912.
Semi-lone-child-avoiding rooted trees are A331934.
Cf. A000081, A001678, A004111, A050381, A214577, A291636, A320230, A320269, A331784, A331872, A331914, A331935, A331966.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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