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 A339193 Matula-Goebel numbers of unlabeled binary rooted semi-identity trees. 5
 1, 4, 14, 86, 301, 886, 3101, 3986, 13766, 13951, 19049, 48181, 57026, 75266, 85699, 199591, 263431, 295969, 298154, 302426, 426058, 882899 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Definition: A positive integer belongs to the sequence iff it is 1, 4, or a squarefree semiprime whose prime indices both already belong to the sequence. A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798. In a semi-identity tree, only the non-leaf branches of any given vertex are distinct. Alternatively, a rooted tree is a semi-identity tree if the non-leaf branches of the root are all distinct and are themselves semi-identity trees. The Matula-Goebel number of an unlabeled 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. LINKS EXAMPLE The sequence of terms together with the corresponding unlabeled rooted trees begins:       1: o       4: (oo)      14: (o(oo))      86: (o(o(oo)))     301: ((oo)(o(oo)))     886: (o(o(o(oo))))    3101: ((oo)(o(o(oo))))    3986: (o((oo)(o(oo))))   13766: (o(o(o(o(oo)))))   13951: ((oo)((oo)(o(oo))))   19049: ((o(oo))(o(o(oo))))   48181: ((oo)(o(o(o(oo)))))   57026: (o((oo)(o(o(oo)))))   75266: (o(o((oo)(o(oo)))))   85699: ((o(oo))((oo)(o(oo)))) MATHEMATICA primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]]; mgbiQ[n_]:=Or[n==1, n==4, SquareFreeQ[n]&&PrimeOmega[n]==2&&And@@mgbiQ/@primeMS[n]]; Select[Range[1000], mgbiQ] CROSSREFS Counting these trees by number of nodes gives A063895. A000081 counts unlabeled rooted trees with n nodes. A111299 ranks binary trees, counted by A001190. A276625 ranks identity trees, counted by A004111. A306202 ranks semi-identity trees, counted by A306200. A306203 ranks balanced semi-identity trees, counted by A306201. A331965 ranks lone-child avoiding semi-identity trees, counted by A331966. Cf. A007097, A061775, A196050, A291636, A331963, A331964. Sequence in context: A327355 A024421 A259353 * A352289 A330465 A202139 Adjacent sequences:  A339190 A339191 A339192 * A339194 A339195 A339196 KEYWORD nonn AUTHOR Gus Wiseman, Mar 14 2021 STATUS approved

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Last modified May 16 13:45 EDT 2022. Contains 353704 sequences. (Running on oeis4.)