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A358578
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Matula-Goebel numbers of rooted trees whose number of leaves equals their number of internal (non-leaf) nodes.
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23
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2, 6, 7, 18, 20, 21, 26, 34, 37, 43, 54, 60, 63, 67, 70, 78, 88, 91, 92, 95, 102, 111, 116, 119, 122, 129, 142, 146, 151, 162, 164, 173, 180, 181, 189, 200, 201, 202, 210, 227, 234, 236, 239, 245, 260, 264, 269, 273, 276, 278, 285, 306, 308, 314, 322, 333, 337
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OFFSET
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1,1
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COMMENTS
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The Matula-Goebel number of a rooted tree is the product of primes indexed by the Matula-Goebel numbers of the branches of its root, which gives a bijective correspondence between positive integers and unlabeled rooted trees.
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LINKS
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FORMULA
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EXAMPLE
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The terms together with their corresponding rooted trees begin:
2: (o)
6: (o(o))
7: ((oo))
18: (o(o)(o))
20: (oo((o)))
21: ((o)(oo))
26: (o(o(o)))
34: (o((oo)))
37: ((oo(o)))
43: ((o(oo)))
54: (o(o)(o)(o))
60: (oo(o)((o)))
63: ((o)(o)(oo))
67: (((ooo)))
70: (o((o))(oo))
78: (o(o)(o(o)))
88: (ooo(((o))))
91: ((oo)(o(o)))
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MATHEMATICA
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MGTree[n_]:=If[n==1, {}, MGTree/@Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
Select[Range[100], Count[MGTree[#], {}, {0, Infinity}]==Count[MGTree[#], _[__], {0, Infinity}]&]
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CROSSREFS
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A034781 counts trees by nodes and height.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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