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Number of semi-lone-child-avoiding rooted trees with at most one distinct non-leaf branch directly under any vertex.
10

%I #9 Feb 09 2020 18:58:00

%S 1,1,1,2,4,6,12,18,33,52,90,142,242,384,639,1028,1688,2716,4445,7161,

%T 11665,18839,30595,49434,80199,129637,210079,339750,550228,889978,

%U 1440909,2330887,3772845,6103823,9878357,15982196,25863454,41845650,67713550,109559443

%N Number of semi-lone-child-avoiding rooted trees with at most one distinct non-leaf branch directly under any vertex.

%C A rooted tree is semi-lone-child-avoiding if there are no vertices with exactly one child unless the child is an endpoint/leaf.

%H Andrew Howroyd, <a href="/A331933/b331933.txt">Table of n, a(n) for n = 1..1000</a>

%H David Callan, <a href="http://arxiv.org/abs/1406.7784">A sign-reversing involution to count labeled lone-child-avoiding trees</a>, arXiv:1406.7784 [math.CO], (30-June-2014).

%H Gus Wiseman, <a href="https://docs.google.com/document/d/e/2PACX-1vS1zCO9fgAIe5rGiAhTtlrOTuqsmuPos2zkeFPYB80gNzLb44ufqIqksTB4uM9SIpwlvo-oOHhepywy/pub">Sequences counting series-reduced and lone-child-avoiding trees by number of vertices.</a>

%H Gus Wiseman, <a href="/A331933/a331933.png">The a(11) = 90 semi-lone-child-avoiding rooted trees with at most one distinct non-leaf branch directly under any vertex.</a>

%F a(n) = 1 + Sum_{i=2..n-2} floor((n-1)/i)*a(i). - _Andrew Howroyd_, Feb 09 2020

%e The a(1) = 1 through a(8) = 18 trees:

%e o (o) (oo) (ooo) (oooo) (ooooo) (oooooo)

%e (o(o)) (o(oo)) (o(ooo)) (o(oooo))

%e (oo(o)) (oo(oo)) (oo(ooo))

%e ((o)(o)) (ooo(o)) (ooo(oo))

%e (o(o)(o)) (oooo(o))

%e (o(o(o))) ((oo)(oo))

%e (o(o(oo)))

%e (o(oo(o)))

%e (oo(o)(o))

%e (oo(o(o)))

%e ((o)(o)(o))

%e (o((o)(o)))

%t sseo[n_]:=Switch[n,1,{{}},2,{{{}}},_,Join@@Function[c,Select[Union[Sort/@Tuples[sseo/@c]],Length[Union[DeleteCases[#,{}]]]<=1&]]/@Rest[IntegerPartitions[n-1]]];

%t Table[Length[sseo[n]],{n,10}]

%o (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

%Y Not requiring lone-child-avoidance gives A320222.

%Y The non-semi version is A320268.

%Y Matula-Goebel numbers of these trees are A331936.

%Y Achiral trees are A003238.

%Y Semi-identity trees are A306200.

%Y Numbers S with at most one distinct prime index in S are A331912.

%Y Semi-lone-child-avoiding rooted trees are A331934.

%Y Cf. A000081, A001678, A004111, A050381, A214577, A291636, A320230, A320269, A331784, A331872, A331914, A331935, A331966.

%K nonn

%O 1,4

%A _Gus Wiseman_, Feb 03 2020

%E Terms a(31) and beyond from _Andrew Howroyd_, Feb 09 2020