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Numbers k such that the k-th standard ordered rooted tree is an identity tree (counted by A032027).
7

%I #5 Nov 14 2022 20:01:14

%S 1,2,3,5,6,7,10,13,17,19,21,33,34,38,39,42,45,49,51,53,65,66,67,81,97,

%T 130,131,133,134,135,145,161,162,177,193,195,209,259,261,262,263,266,

%U 269,289,290,305,321,322,353,387,389,401,417,513,517,518,519,522

%N Numbers k such that the k-th standard ordered rooted tree is an identity tree (counted by A032027).

%C We define the n-th standard ordered rooted tree to be obtained by taking the (n-1)-th composition in standard order (graded reverse-lexicographic, A066099) as root and replacing each part with its own standard ordered rooted tree. This ranking is an ordered variation of Matula-Goebel numbers, giving a bijective correspondence between positive integers and unlabeled ordered rooted trees.

%C A rooted identity tree is an unlabeled rooted tree with no repeated branches directly under the same root.

%H Gus Wiseman, <a href="https://docs.google.com/document/d/e/2PACX-1vTCPiJVFUXN8IqfLlCXkgP15yrGWeRhFS4ozST5oA4Bl2PYS-XTA3sGsAEXvwW-B0ealpD8qnoxFqN3/pub">Statistics, classes, and transformations of standard compositions</a>

%e The terms together with their corresponding ordered rooted trees begin:

%e 1: o

%e 2: (o)

%e 3: ((o))

%e 5: (((o)))

%e 6: ((o)o)

%e 7: (o(o))

%e 10: (((o))o)

%e 13: (o((o)))

%e 17: ((((o))))

%e 19: (((o))(o))

%e 21: ((o)((o)))

%e 33: (((o)o))

%e 34: ((((o)))o)

%e 38: (((o))(o)o)

%e 39: (((o))o(o))

%e 42: ((o)((o))o)

%e 45: ((o)o((o)))

%t stc[n_]:=Differences[Prepend[Join@@Position[Reverse[IntegerDigits[n,2]],1],0]]//Reverse;

%t srt[n_]:=If[n==1,{},srt/@stc[n-1]];

%t Select[Range[100],FreeQ[srt[#],_[__]?(!UnsameQ@@#&)]&]

%Y These trees are counted by A032027.

%Y The unordered version is A276625, counted by A004111.

%Y A000081 counts unlabeled rooted trees, ranked by A358378.

%Y A358371 and A358372 count leaves and nodes in standard ordered rooted trees.

%Y A358375 ranks ordered binary trees, counted by A126120.

%Y Cf. A001263, A004249, A005043, A063895, A358373, A358376, A358377.

%K nonn

%O 1,2

%A _Gus Wiseman_, Nov 14 2022