A193776 - OEIS

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A193776

Number of signed permutations of length n invariant under the reverse complement and avoiding (-2, -1), (-2, 1), (2, -1).

1, 2, 3, 5, 12, 17, 65, 80, 473, 527 (list; graph; refs; listen; history; text; internal format)

	OFFSET	`0,2`
	LINKS	`Table of n, a(n) for n=0..9.`
	FORMULA	`a(2k) = 2^k k! + \sum_{j=0}^{k-1}(k-j-1)! a(2j)` `a(2k+1) = (2^k + 1)k! + \sum_{j=0}^{k-1}(k-j-1)! a(2j+1)`
	EXAMPLE	`For n = 2, the permutations are (1, 2), (2, 1), (-1, -2), (-2, -1).`
	CROSSREFS	`Sequence in context: A158936 A002139 A140489 * A051915 A064688 A089891` `Adjacent sequences: A193773 A193774 A193775 * A193777 A193778 A193779`
	KEYWORD	`nonn`
	AUTHOR	`Andy Hardt, Aug 04 2011`
	STATUS	`approved`

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Last modified July 31 03:36 EDT 2014. Contains 245079 sequences.