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A024255 a(n) = A000111(n-1)*n/2. +0
3
0, 1, 4, 48, 1088, 39680, 2122752, 156577792, 15230058496, 1888788086784, 290888851128320, 54466478584365056, 12185086638082228224, 3209979242472703787008, 983522422455215438430208 (list; graph; listen)
OFFSET

0,3

COMMENT

Number of cyclically alternating permutations of length 2n.

REFERENCES

N. D. Elkies, On the sums Sum_{k = -infinity .. infinity} (4k+1)^(-n), Amer. Math. Monthly, 110 (No. 7, 2003), 561-573.

G. Kreweras, Les preordres totaux compatibles avec un ordre partiel. Math. Sci. Humaines No. 53 (1976), 5-30.

LINKS

N. D. Elkies, On the sums Sum((4k+1)^(-n),k,-inf,+inf)

FORMULA

G.f.: tan(x).x/2.

a(n) = 2^(n-1)*(2^n-1)*|B_n|.

a(n) = A000111(n-1)*n/2.

MATHEMATICA

Tan[x]*x/2 (* Even Part *)

CROSSREFS

Cf. A009752.

Adjacent sequences: A024252 A024253 A024254 this_sequence A024256 A024257 A024258

Sequence in context: A052714 A138448 A071221 this_sequence A013145 A013150 A011266

KEYWORD

nonn

AUTHOR

R. H. Hardin (rhh(AT)cadence.com)

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Last modified October 12 15:26 EDT 2008. Contains 144830 sequences.


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