id:A089848 - OEIS Search Results

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Number of fixed points in range [A014137(n-1)..A014138(n-1)] of permutation A089851/A089853.

+0
6

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 (list; graph; listen)

	OFFSET	`0,3`
	COMMENT	`The number of n-node binary trees fixed by the corresponding automorphism(s).`
	LINKS	`A. Karttunen, C-program for computing the initial terms of this sequence`
	FORMULA	`a(0)=1, a(n) = A000108(n-1) if (n mod 3)=0 or 1, and a(n) = A000108(n-1)+A000108((n-2)/3) if (n mod 3)=2.`
	MAPLE	A089848 := n -> A000108(n-1)+`if`((2 = (n mod 3)), A000108((n-2)/3), 0);
	PROGRAM	`(Scheme) (define (A089848 n) (cond ((zero? n) 1) (else (+ (A000108 (- n 1)) (if (= 2 (modulo n 3)) (A000108 (/ (- n 2) 3)) 0)))))`
	CROSSREFS	`Sequence in context: A032097 A112709 A098888 this_sequence A033550 A032130 A019099` `Adjacent sequences: A089845 A089846 A089847 this_sequence A089849 A089850 A089851`
	KEYWORD	`nonn,easy`
	AUTHOR	`Antti Karttunen (His_Firstname.His_Surname(AT)iki.fi), Nov 29 2003`

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Last modified August 19 23:53 EDT 2008. Contains 142930 sequences.

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