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 A331873 Matula-Goebel numbers of semi-lone-child-avoiding locally disjoint rooted trees. 14
 1, 2, 4, 6, 8, 9, 12, 14, 16, 18, 24, 26, 27, 28, 32, 36, 38, 46, 48, 49, 52, 54, 56, 64, 69, 72, 74, 76, 81, 86, 92, 96, 98, 104, 106, 108, 112, 122, 128, 138, 144, 148, 152, 161, 162, 169, 172, 178, 184, 192, 196, 202, 206, 207, 208, 212, 214, 216, 224, 243 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS First differs from A331936 in having 69, the Matula-Goebel number of the tree ((o)((o)(o))). A rooted tree is semi-lone-child-avoiding if there are no vertices with exactly one child unless the child is an endpoint/leaf. Locally disjoint means no child of any vertex has branches overlapping the branches of any other (inequivalent) child of the same vertex. 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 distinct prime indices are pairwise coprime and already belong to the sequence, where a singleton is always considered to be pairwise coprime. A prime index of n is a number m such that prime(m) divides n. LINKS Table of n, a(n) for n=1..60. David Callan, A sign-reversing involution to count labeled lone-child-avoiding trees, arXiv:1406.7784 [math.CO], (30-June-2014). Gus Wiseman, Sequences counting series-reduced and lone-child-avoiding trees by number of vertices. EXAMPLE The sequence of all semi-lone-child-avoiding locally disjoint 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)) 24: (ooo(o)) 26: (o(o(o))) 27: ((o)(o)(o)) 28: (oo(oo)) 32: (ooooo) 36: (oo(o)(o)) 38: (o(ooo)) 46: (o((o)(o))) 48: (oooo(o)) 49: ((oo)(oo)) MATHEMATICA msQ[n_]:=n==1||n==2||!PrimeQ[n]&&(PrimePowerQ[n]||CoprimeQ@@PrimePi/@First/@FactorInteger[n])&&And@@msQ/@PrimePi/@First/@FactorInteger[n]; Select[Range[100], msQ] CROSSREFS Not requiring lone-child-avoidance gives A316495. A superset of A320269. The semi-identity tree case is A331681. The non-semi version (i.e., not containing 2) is A331871. These trees counted by vertices are A331872. These trees counted by leaves are A331874. Not requiring local disjointness gives A331935. The identity tree case is A331937. Cf. A007097, A050381, A061775, A196050, A291636, A302696, A316473, A316696, A316697, A331680, A331682, A331683, A331687, A331934. Sequence in context: A138969 A161819 A331935 * A331936 A364123 A244799 Adjacent sequences: A331870 A331871 A331872 * A331874 A331875 A331876 KEYWORD nonn AUTHOR Gus Wiseman, Feb 02 2020 STATUS approved

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Last modified July 15 11:27 EDT 2024. Contains 374332 sequences. (Running on oeis4.)