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A331935 Matula-Goebel numbers of semi-lone-child-avoiding rooted trees. 16
1, 2, 4, 6, 8, 9, 12, 14, 16, 18, 21, 24, 26, 27, 28, 32, 36, 38, 39, 42, 46, 48, 49, 52, 54, 56, 57, 63, 64, 69, 72, 74, 76, 78, 81, 84, 86, 91, 92, 96, 98, 104, 106, 108, 111, 112, 114, 117, 122, 126, 128, 129, 133, 138, 144, 146, 147, 148, 152, 156, 159 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
A rooted tree is semi-lone-child-avoiding if there are no vertices with exactly one child unless the child is an endpoint/leaf.
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.
Consists of one, two, and all nonprime numbers whose prime indices already belong to the sequence, where a prime index of n is a number m such that prime(m) divides n.
LINKS
David Callan, A sign-reversing involution to count labeled lone-child-avoiding trees, arXiv:1406.7784 [math.CO], (30-June-2014).
EXAMPLE
The sequence of all semi-lone-child-avoiding rooted trees together with their Matula-Goebel numbers begins:
1: o
2: (o)
4: (oo)
6: (o(o))
8: (ooo)
9: ((o)(o))
12: (oo(o))
14: (o(oo))
16: (oooo)
18: (o(o)(o))
21: ((o)(oo))
24: (ooo(o))
26: (o(o(o)))
27: ((o)(o)(o))
28: (oo(oo))
32: (ooooo)
36: (oo(o)(o))
38: (o(ooo))
39: ((o)(o(o)))
42: (o(o)(oo))
The sequence of terms together with their prime indices begins:
1: {} 46: {1,9} 98: {1,4,4}
2: {1} 48: {1,1,1,1,2} 104: {1,1,1,6}
4: {1,1} 49: {4,4} 106: {1,16}
6: {1,2} 52: {1,1,6} 108: {1,1,2,2,2}
8: {1,1,1} 54: {1,2,2,2} 111: {2,12}
9: {2,2} 56: {1,1,1,4} 112: {1,1,1,1,4}
12: {1,1,2} 57: {2,8} 114: {1,2,8}
14: {1,4} 63: {2,2,4} 117: {2,2,6}
16: {1,1,1,1} 64: {1,1,1,1,1,1} 122: {1,18}
18: {1,2,2} 69: {2,9} 126: {1,2,2,4}
21: {2,4} 72: {1,1,1,2,2} 128: {1,1,1,1,1,1,1}
24: {1,1,1,2} 74: {1,12} 129: {2,14}
26: {1,6} 76: {1,1,8} 133: {4,8}
27: {2,2,2} 78: {1,2,6} 138: {1,2,9}
28: {1,1,4} 81: {2,2,2,2} 144: {1,1,1,1,2,2}
32: {1,1,1,1,1} 84: {1,1,2,4} 146: {1,21}
36: {1,1,2,2} 86: {1,14} 147: {2,4,4}
38: {1,8} 91: {4,6} 148: {1,1,12}
39: {2,6} 92: {1,1,9} 152: {1,1,1,8}
42: {1,2,4} 96: {1,1,1,1,1,2} 156: {1,1,2,6}
MATHEMATICA
mseQ[n_]:=n==1||n==2||!PrimeQ[n]&&And@@mseQ/@PrimePi/@First/@FactorInteger[n];
Select[Range[100], mseQ]
CROSSREFS
The enumeration of these trees by leaves is A050381.
The locally disjoint version A331873.
The enumeration of these trees by nodes is A331934.
The case with at most one distinct non-leaf branch of any vertex is A331936.
Lone-child-avoiding rooted trees are counted by A001678.
Matula-Goebel numbers of lone-child-avoiding rooted trees are A291636.
Sequence in context: A189170 A138969 A161819 * A331873 A331936 A364123
KEYWORD
nonn
AUTHOR
Gus Wiseman, Feb 03 2020
STATUS
approved

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Last modified July 22 21:11 EDT 2024. Contains 374544 sequences. (Running on oeis4.)