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1, 1, 2, 2, 5, 15, 42, 132, 431, 1430, 4862, 16801, 58786, 208012, 742914, 2674440, 9694845, 35357712, 129644790, 477638700, 1767263322, 6564120420, 24466267020, 91482564069, 343059613650, 1289904147324, 4861946402882, 18367353072152
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OFFSET
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0,3
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COMMENTS
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The number of n-node binary trees fixed by the corresponding automorphism(s).
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LINKS
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FORMULA
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MAPLE
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PROG
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(Scheme) (define (A089848 n) (cond ((zero? n) 1) (else (+ (A000108 (- n 1)) (if (= 2 (modulo n 3)) (A000108 (/ (- n 2) 3)) 0)))))
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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